Heredity and Sex (1913) 4
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Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.
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Chapter IV Secondary Sexual Characters and their Relation to Darwin's Theory of Sexual Selection
In his "Origin of Species" Darwin has defined Sexual Selection as depending on a struggle between the individuals of one sex, generally the males, for the possession of the other sex. The result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection. Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny. But in many cases, victory depends not so much on general vigor, as on having special weapons, confined to the male sex. A hornless stag or spurless cock would have a poor chance of leaving numerous offspring. Sexual selection, by always allowing the victor to breed, might surely give indomitable courage, length to the spur, and strength to the wing to strike in the spurred leg, in nearly the same manner as does the brutal cock-fighter by the careful selection of his best cocks."
Darwin continues: Amongst birds, the contest is often of a more peaceful character. All those who have attended to the subject, believe that there is the severest rivalry between the males of many species to attract, by singing, the females. The rock-thrush of Guiana, birds of paradise, and some others, congregate, and successive males display, with the most elaborate care, and show off in the best manner, their gorgeous plumage ; they likewise perform strange antics before the females, which, standing by as spectators, at last choose the most attractive partner/'
Here we have two different pictures, each of which attempts to give an account of how certain differences between the sexes have arisen — differences that we call "secondary sexual characters."
On the one hand we deal with a contest between the males ; on the other with choice by the female. The modus operandi is also different. After battle the successful male takes his pick of the females. If the scheme is to work, he must choose one that will leave the most offspring.
On the other hand, we have the tourney of love. The males show off"; the females stand by spellbound and at last choose the most attractive partner."
Now, concerning this display of the males, I beg leave to quote a paragraph from Wallace's "Natural Selection and Tropical Nature" :
"It is a well-known fact that when male birds possess any unusual ornaments, they take such positions or perform such evolutions as to exhibit them to the best advantage while endeavoring to attract or charm the females, or in rivalry with other males. It is therefore probable that the wonderfully varied decorations of humming-birds, whether burnished breastshields, resplendent tail, crested head, or glittering back, are thus exhibited ; but almost the only actual observation of this kind is that of Mr. Belt, who describes how two males of the Florisuga mellivora displayed their ornaments before a female bird. One would shoot up like a rocket, then, suddenly expanding the snow-white tail like an inverted parachute, slowly descend in front of her, turning around gradually to show off both back and front. The expanded white tail covered more space than all the rest of the bird, and was evidently the grand feature of the performance. Whilst one was descending the other would shoot up and come slowly down expanded."
There is just a suspicion in my. mind that these males were otherwise engaged, for while I know nothing about the habits of these humming birds I find on the next page of " Tropical Nature " this statement :
^'Mr. Gosse also remarks: ^All the hummingbirds have more or less the habit, when in flight, of pausing in the air and throwing the body and tail into rapid and odd contortions. This is most observable in Polytmus, from the effect that such motions have on the long feathers of the tail. That the object of these quick turns is the capture of insects, I am sure, having watched one thus engaged.' "
If what I have just said implies that I take a lighthearted or even facetious attitude toward Darwin's theory, I trust that my position will not be misunderstood. Darwin brought together in his book on the ^'Descent of Man" a mass of interesting observations for which he suggested a new theory. No one can read his wonderful book without the keenest interest, or leave it without high admiration for the thoroughness with which the subject is treated ; for the ingenuity and skill with which the theory is applied to the facts, and, above, all, admiration for the moderation, modesty, and honesty with which objections to the theory are considered.
I will let no one admire Darwin more than I admire Darwin. But while affection and respect and honor are the finest fruits of our relation to each other, we cannot let our admiration for the man and an ever ready recognition of what he has done for you and for me prejudice us one whit in favor of any scientific theory that he proposes. For in Science there is no authority ! We should of course give serious consideration to any theory proposed by a man of such wide experience and trained judgment as Darwin ; but he himself, who broke all the traditions of his race, would be the first to disclaim the value of evidence accepted on authority.
From the definition of sexual selection with which we started it may be said that Competition and Courtship stand for the two ways in which Darwin supposes the secondary sexual characters to have arisen.
Competition amongst the males is only a form of natural selection, as Darwin himself recognized (if we leave out of account the further assumption that the victor chooses his spoils). We may dismiss this side of the problem as belonging to the larger field of natural selection, and give our attention mainly to those secondary sexual characters that Darwin supposes to have arisen by the female choosing the more ornamented suitor.
I shall first bring forward some of the more striking examples of secondary sexual characters in the animal kingdom. These characters are confined almost exclusively to three great groups of animals — Insects, Spiders, and Vertebrates. There are a few scattered instances found in other groups, but they are rare. In the lowest groups they are entirely absent, and are
Fig. 52 — Four species of beetles in which the male (to the left) has horns which are absent in the female (to the right). (After Darwin.)
not found at all in plants ; or rather, if characteristic differences exist in plants, they are not called by this name — for plants cannot see or move and therefore cannot court each other.
In fact, sight in the sense of forming visual pictures can occur only when eyes are well developed. This may be taken to score a point in favor of Darwin's hypothesis.
Fig. 53. — Male (to left) with long eye stalks and female (to right) of a fly, Achia longividens. (After Wood.)
In the group of insects the most noticeable differences occur in the butterflies and moths, and in flies. A few cases are found in the beetles and bugs. The male cicada's shrill call is supposed to attract the females. The males of certain beetles have horns — the female lacks them (Fig. 52).
Fig. 54. — Male to left with horns and female to right without horns of a fly, Elaphomyia. (After Wood.)
In a genus of flies the eyes are stalked, and the eyes of the male have stalks longer than those of the female (Fig. 53). In another genus of flies there are horns on the head like the antlers of the stag (Fig. 54).
In the spiders the adult males are sometimes verysmall in comparison with the females (Fig. 55). The size difference may be regarded as a secondary sexual character. Darwin points out, since the male is sometimes devoured by the female (if his attentions are not desired) , that his small size may be an adaptation in order that he may more readily escape. But the point may be raised as to whether he is small in order to escape ; or whether he is eaten because he is small. In one of our native spiders, Habrocestum splendida, the adult males and females are conspicuously different in color — the male more highly colored than the female. In another native species, Maevia vittata, there are two kinds of males, both colored differently from the female.
Fig. 55. — Male (to left) and female (to right) of a spider, Argiope aurelia. (From " Cambridge Natural History.")
Passing over the groups of fishes and reptiles in which some striking cases of differences between the sexes occur, we come to the birds, where we find the best examples of secondary sexual characters.
Fig. 56. — Superb bird of paradise. (After Elliot.)
In the white-booted humming bird (Fig. 14) two of the tail feathers of the male are drawn out, their shafts denuded of the vanes except at the tip where the feather ends in a broad expansion.
In the great bird of paradise, of the Aru Islands (Fig. 13), the male has wonderful plumes arising from the sides that can be erected to produce a gorgeous display.
The female is modestly clothed. In the male of the superb bird of paradise (Fig. 56), the mantle behind the neck, when erected, forms a striking ornament ; and on the breast there is a brilliant metallic shield. In the six-shafted bird of paradise (Fig. 57) the male has on its head six feathers with wiry shafts,
Fig. 57. — Six-shafted bird of paradise. (After Elliot.)
ornaments that occur in no other birds. In the king bird of paradise there are^ remarkable fans at the sides of the body of the male that can be expanded. The feathers of the fan are emerald-tipped. The two middle feathers of the tail are drawn out into wires" with a green web at one side of the tip.
In mammals, secondary sexual differences are very common, although starthng differences in color are rather rare. In the male the coat of fur is often darker than that of the female.
In many deer the antlers are present in the male alone. In Steller's sea-lion the male is much larger and stronger than the female. In a race of the Asiatic elephant the male has tusks much larger than those of the female.
Fig. 58. — Wilson's phalarope, female (in center), male (to right and behind). A bird in winter plumage is at the left. (From Eaton, "Birds of New York.")
If we fix our attention exclusively on these remarkable cases where differences between the sexes exist, we get a one-sided impression of the development of ornamentation and color differences in animals. We must not forget that in many cases males and females are both highly colored and exactly alike. We forget the parrots, the cockatoos, the kingfishers, the crowned pigeons, toucans, lories, and some of the starlings; the ^'brilhant todies" and the '^sluggish jacamars" whose brilliant metallic golden-green breasts rival those of the humming-birds; we forget the zebras, the leopards ; the iridescent interiors of the shells of many mollusks ; the bright reds and purples of starfish, worms, corals, sea anemones, the red, yellow, and green sponges, and the kaleidoscopic effect of the microscopic radiolarians ; — a brilliant array of color.
Fig. 59. — (A), female of a copepod, Calocalanus plumosus. (B), a female of Calocalanus parvus. (C) male of last.
In the egret both males and females have remarkable nuptial plumes, which, had they been present in one sex alone, would have been classified as secondary sexual characters. It does not appear that selection had anything to do with their creation.
Our common screech owl exists in two colored types sharply separated. No one is likely to ascribe these differences to sexual selection, yet if one sex had been red and the other gray, the difference would have been put down to such selection. There are also cases like the phalarope, shown in Fig. 58, where the female is more highly ornamented than the male. In fact, for these cases, Darwin supposed that the males select the females ; and in support of this view he points out that the females are more active, while the male concerns himself with the brooding of the eggs. In some of the marine copepods female ornamentation is carried to even a higher point. In Calocalanus plumosus the female has one of the tail setae drawn out into a long feather-like structure (Fig. 59). In another species, C. parva, all eight setae of the tail of the female are feather-like (Fig. 59, B), while the male (Fig. 59, C) lacks entirely these "ornaments."
In some butterflies also, two, three, or more types of females are known, but only one male type. I shall have occasion later to consider this case.
Courtship
The theory of sexual selection hinges in the first place on whether the female chooses amongst her suitors.
It has been objected that the theory is anthropomorphic — it ascribes to beetles, butterflies, and birds the highly developed esthetic sense of man. It has been objected that the theory leaves unexplained the development of this esthetic sense itself, for unless the female kept in advance of the male it is not self-evident why she should go on selecting the more highly ornamented. If she has advanced esthetically, what has brought it about ? In answer to this last question Allen suggests that if the word conspicuousness is substituted for the word beauty, the objection may to some extent be met. The more conspicuous male would be more likely to attract attention and be selected.
It has been pointed out that there is more than a suspicion that the contests of the males for the females are sham affairs. They are like certain duels. There is seldom any one hurt. There are very few records of injured males, but many accounts of tremendous battles. And he who fights and runs away will live to mate another day.
It is clear, I think, that the case against the theory must rest its claims on actual evidence rather than on arguments or poetry pro or con. Darwin admitted that the evidence was meager. Since his time something more has been done. Let us consider some of this new evidence.
It will be conceded, I think, that Alfred Wallace, through his wide experience with animals in their native haunts, is in a position to give weighty evidence concerning the behavior of animals. He was with Darwin a co-discoverer of the theory of Natural Selection and cannot be supposed to be prejudiced against the selection principle. Yet Wallace has from the beginning strongly opposed the theory of sexual selection. Let me quote him :
Referring to Darwin's theory of Sexual Selection —
^'I have long held this portion of Darwin's theory to be erroneous — and have held that the primary cause of sexual diversity of color was the need of protection, repressing in the female those bright colors which are normally produced in both sexes by general laws."
Again, Wallace says: To conscious sexual selection — that is, the actual choice by the females of the more brilliantly colored males or the rejection of those less gaily colored — I believe very little if any effect is directly due. It is undoubtedly proved that in birds the females do sometimes exert a choice ; but the evidence of this fact, collected by Mr. Darwin ('Descent of Man,' chap, xiv), does not prove that color determines that choice, while much of the strongest evidence is directly opposed to this view."
Again, Wallace says: "Amid the copious mass of facts and opinions collected by Mr. Darwin as to the display of color and ornaments by the male birds, there is a total absence of any evidence that the females, as a rule, admire or even notice this display. The hen, the turkey, and the peafowl go on feeding, while the male is displaying his finery ; and there is reason to believe that it is his persistency and energy rather than his beauty which wins the day."
Hudson, who has studied the habits of birds in the field, asks some very pertinent questions in connection with their performances of different kinds. "What relation to the passion of love and to the business of courtship have these dancing and vocal performances in nine cases out of ten ? In such cases, for instance, as that of the scissor-tail tyrant-bird, and its pyrotechnic displays, when a number of couples leave their nests containing eggs and young to join in a wild aerial dance ; the mad exhibition of grouped wings ; the triplet dances of the spur- winged lapwing, to perform which two birds already mated are compelled to call in a third bird to complete the set ; the harmonious duets of the oven-birds and the duets and choruses of nearly all the wood-hewers, and the wing-slapping aerial displays of the whistling widgeons, — will it be seriously contended that the female of this species makes choice of the male able to administer the most vigorous and artistic slaps?"
He continues: How unfair the argument is, based on these carefully selected cases, gathered from all regions of the globe, and often not properly reported, is seen when we turn to the book of nature and closely consider the habits and actions of all the species inhabiting any one district." Hudson concludes that he is convinced that anybody who will note the actions of animals for himself will reach the conviction, that conscious sexual selection on the part of the female is not the cause of music and dancing performances in birds, nor of the brighter colors and ornaments that distinguish the male."
In the spiders Mr. and Mrs. Peckham have described in detail the courtship of the males. They believe that his antics are specifically intended to attract the female. They point out that his contortions are of such a sort that his brightest spots are turned toward the female. But, as he makes in any case a hundred twists and turns, there is some danger of misinterpreting his poses. Montgomery, who has studied spiders of other groups, reaches the conclusion that here the male is contorted through fear of the female. The male goes through some of the same turns if approached by another male. The courtship of the male spider is, he thinks, a motley of fear, desire, and general excitement.
The evidence that the Peckhams have given, even if taken to mean that the motions of the male attract the attention of the female, — and I can see no reason why this may not be the case, — fails nevertheless to show that the female selects, when she has a chance, the more highly colored male.
Mayer, and Mayer and Soule have made many experiments with moths. The moth promethea, Callosamia promethea, is distinctly sexually dimorphic, as shown in Fig. 60. Mayer's experiments show that the male finds the female entirely by the sense of smell. The wings of some 300 males were painted with scarlet or green. They mated as often as did the normal male with which they competed.
Fig. 60. — Above, Callosamia promethia, male to left, female to right. Below Porthetria dispar, male to left, female to right.
Where the wings of males were stuck on the female in place of her own wings, no disturbance in the mating was observed. Conversely, normal females accepted males with female wings as readily as they accepted normal males.
In the gipsy moth (Porthetria dispar), the male is brown and the female white (Fig. 60). Here again it was found that the males are guided solely by the odor of the female.
The silkworm moth is also sexually dimorphic. Kellogg has shown that males with blackened eyes find a female with as much precision as does a moth with normal eyes.
If the antennae are cut off, however, the male can not find the female unless by accident he touches her. He then mates. The female has scent glands whose odor excites the male with normal antennae even at some distance. Chemotaxis and contact are the active agents in mating. The eyes do little or nothing.
Andrews has found that touch determines mating in the crayfish. Pearse has obtained similar results. Chidester has shown the same thing for crabs. Holmes found this kind of behavior in Amphipoda. Fielde and Wheeler have also found that in ants sex-discrimination is through smell or by what Forel calls contact-odors.
Montgomery and Porter recognize touch as the most important factor in mating in spiders. Petrunkewitsch has shown that in the hunting spider vision also helps the sexes to find each other. Tower has found that contact or odor rather than sight is the important condition in mating in leptinotarsa.
I am able to give the unpublished results of A. H. Sturtevant on the mating of the fruit fly, drosophila. The male carries on an elaborate courtship in the sense that he circles around the female, throws out one wing, then the other, and shows other signs of excitement. The male has sex combs on his fore legs, the female lacks them. Lutz cut them off and gave the female a choice between such a male and a normal male. One was chosen as often as the other. The wings of the male and female are wonderfully iridescent. Sturtevant cut off the wings of a male and matched him against a normal male. The female showed no marked preference. The converse experiment, when a clipped female competed with a normal female, showed no selection on the part of the males.
If instead of allowing two males (a normal and a clipped) to compete for one female, a female is given to each male separately, and the interval before mating is noted, it is found that on an average this interval is 18 minutes for the normal and 40 minutes for the clipped. If any such difference existed in the first case, when the two males were competing, we should expect a much greater selection in favor of the normal male than was actually found. This would seem to mean that the female is more quickly aroused by the normal male, and hence when both males are present she will accept the clipped male more quickly than when he alone is present. This suggests that normal courtship precipitates copulation.
In the following experiments the female was offered a choice between a new type (mutant) with white eyes and a normal male. Conversely, the white-eyed female had a hke alternative. The evidence shows that the more vigorous male — the red-eyed male — is more successful.
Since vision itself is here involved, for the whiteeyed flies are probably partly blind, the observations
RED VERSUS WHITE EYES.
GRAY VERSUS YELLOW COLOR.
Red J White d Red 9
Red 9 - 54 White 9-82
I Red ? -40 White ? - 93
Red d" - 53 White cf - 14
Gray cT
j Gray ? - 25 I Yellow ? - 31
V 11^ ^ I Gray ? - 12 Yellowed I Yellow? -30
I Gray J - 60 I Yellow d" - 12
Gray ?
White? 1^^^^ "^2
White V I ^J^J^^ ^ _ jg
Yellow? IST^ "^f
I Yellow cT - 8
NORMAL VERSUS CLIPPED WINGS.
GRAY-WHITE VERSUS YELLOW
WHITE.
, o [ Clipped cf - 51
Normal ? ^ ^.t i , r--,
[ Normal J - 67
Normal d"
Clipped ? - 27 Normal ? - 25
Gray J Gray ?
f Gray ?
-11
Yellow ? - 4
[ Gray cf - 21 Yellow d" - 3
(
were repeated with a new type that had yellow wings. The gray male is more successful and the yellow females less resistant. The results are in accord with the assumption that greater vigor is an important factor in success.
The following mating bears on this point. Sturtevant used in competition a red- and a vermilioneyed male. The latter seems as vigorous as is the red-eyed type. The results were :
Red ?
Red S Vermilion
11 14
showing that the red-eyed male has no advantage when the males are equally vigorous.
This evidence, taken as a whole, seems to me to show with some probability that sight plays a minor role in courtship. It is so inferior to vigor, to the sense of smell and to touch in the lower animals at least, that it is very questionable whether it has had anything more to do with mating than helping the sexes find each other.
Vigor and Secondary Sexual Characters
We have seen that Darwin himself has stated explicitly that unless the secondary sexual characters are associated with greater vigor, or productivity, nothing can be accomplished.
It will be recalled that Wallace, who disbelieved in Darwin's theory of sexual selection, attempted to account for the appearance of secondary sexual characters on the ground of the greater vigor of the male (he sometimes says vitality and again activity of the male) at the breeding season. The vigor is assumed to be associated with the development of the sex glands at this time. This may be admitted, but whether the vigor is the result of the sex glands, or the sex glands of the vigor, is a nice point that I shall not try to decide. It may appear that Wallace's view is in part justified from the facts that we have examined. But I do not think so. In the first place, he attempts the impossible task of explaining the outgrowths and colors that appear in special regions by the local activity of the muscles (for example) in those regions. The facts before us do not support any such interpretation. The Peckhams easily overturn his argument, as applied to spiders.
Second, in birds, to which Wallace mainly refers, the sex glands of the male do not affect the secondary sexual characters of the male, while the sex glands of the female suppress these characters.
Wallace's theory leaves out of account the hereditary factor that is also present and which acts quite apart from the physiological effects of the sex glands.
Cunningham, who has more recently written on the same subject, accepts the hormone hypothesis as the basis for all cases of secondary sexual characters. But he fails to make good his view when it is applied to insects, for.reasons that we shall take up later. He is especially concerned, however, in the attempt to make plausible his own hypothesis that secondary sexual characters have arisen through the use of the parts, or through special nervous or blood suppHes to certain locahties of the body which become suffused during sexual excitement. In both cases he thinks the increased local activity will cause the cells to produce hormones that will be dispersed throughout the body, and absorbed by other cells. The germ-cells will in this way get their share and carry over the hormone to the next generation.
Cunningham forgets one important point. If these imaginary hormones can get out of cells and into germcells, they can get out of the germ-cells again. Hence in the long period of embryonic and juvenile existence through which the individual passes before the secondary sexual characters appear they would surely be lost from the body like any other ordinary hormone.
Continuous Variation as a Basis for Selection
And now let us turn to an entirely different aspect of the matter. What could selection do, admitting that selection may take place. For fifty years it has been taken for granted that by selecting a particular kind of individual the species will move in the direction of selection.
Fig. 61 — I. Diagram of five pure lines of beans {A, B, C, D, and E) and a population formed by their union, A-E. II. Diagrams illustrating a pure line of beans and two new biotypes derived from it. 1 he upper diagram indicates the original biotype ; the second and third diagram indicate the elongated (narrower) and shorter (broader) type of beans X indicates the average class of the original biotype. (After Johannsen.)
A few examples will bring the matter before us. If we take a peck of beans and put all of those of one size in one cylinder and those of other sizes in other cylinders, and place the cylinders in a row, we get a result like that in Fig. 61, A-E. If we imagine a Une joining the tops of the beans, the line gives a curve like that shown in Fig. 62. This is known as the curve of probability. The curve can be, of course, most readily made by making the measurements directly. Most individuals of such a population will have the character developed to the degree represented by the highest point in the curve. Now if two individuals standing at one side (let us say with the character in question better developed than the average) become the parent of the next generation, their offspring will make a hew curve that has moved, so to speak, in the direction of selection (Fig. 63).
Fig. 62. — The normal binomial curve or the "ideal curve" of distribution. At the base line, the directions from the average value (o) are indicated with the standard deviation (o") as unity. (After Johannsen.)
If again two more extreme individuals are selected, another step is taken. The process is assumed to go on as long as the selection process is maintained.
So the matter stood until a Danish botanist, Johannsen, set seriously to work to test the validity of the assumption, using a race of garden beans for his measurements. He discovered in the first place that popu
FiG. 63. — Schematic representation of the type-shifting effect of selection from the point of view of Galton's reversion theory. The * marks the point on the curves of A, Ai, A2 from which the selection is supposed to be made. (Goldschmidt.)
lations are made up of a number of races or pure hues." When we select in such a population we sort out and separate its constituent races, and sooner or later under favorable conditions can get a pure race. Selection has created nothing new ; it has picked out a particular preexisting race from a mixed population.
Johannsen has shown that within a pure line selection produces no effect, since the offspring form the same group with the same mode as the group from which the parents came. The variabihty within the pure lines is generally ascribed to environmental influences which are recurrent in each generation. The germplasm is homogeneous for all members of the pure hne, while in a mixed population the germ-plasm is not the same for all individuals.
Darwin himself saw this to some extent, for he has repeatedly pointed out that selection depends on the materials offered to it by variation; that in itself it can produce nothing. Yet from Darwin to Johannsen the teaching of the post-Darwinians has been such as to lead most people to believe that selection is a causative or creative principle that will explain the progressive development of animals and plants.
Discontinuous Variation or Mutation as a Basis for Selection
The second great movement since Darwin has been to show that hereditary variations do not give a continuous series but a discontinuous one. Bateson and De Vries brought forward some twelve years ago evidence, in favor of this view, that has gone on increasing in volume at an amazing rate.
I cannot attempt to discuss this evidence here, but I may point out the bearing of the new point of view on the meaning of secondary sexual characters.
In a number of butterflies there occur two or three or even more different kinds of females. One of the most remarkable cases of the kind is that of Papilio polytes that lives in India and Ceylon. It has a single male type and three types of females (Fig. 64).
Wallace, who first observed that the three types of female belong to one male type, argued that two of these three types owe their origin to their resemblance to butterflies of other species that are protected, namely, Papilio aristolochia and P. hector. These
Fig. 64. — Papilio polytes ; male (upper left) and three types of female (to right). The "models," which two of these females are supposed to "mimic," are shown to their left.
two feed on the poisonous plant aristolochia and are said to be unpalatable. The two aberrant types of P. polytes bearing a close resemblance to these two species have been dubbed the hector form and the aristolochia form.
Wallace, and those who adhere to the same view, beUeve that the resemblance of the model and the mimic has come about through the accumulation of minute variations which have survived as a result of their advantages. In a word, the process of natural selection is assumed to have gradually brought about the evolution of these two new types of females.
This case has been recently examined by Punnett.
Punnett says that while in cabinet specimens the resemblance between the model and the mimic is remarkably close, yet in the living animals, with their wings spread out, the resemblance is less marked, especially the resemblance between the hector model and the polytes mimic. At a distance of a few yards the difference between the two is easily seen.
When flying the differences are very apparent. ' ' The mode of flight of P. polytes is similar for all three fdrms, and is totally distinct from that of P. hector and P. aristolochia.'^ In flight the latter pursue an even course, while the polytes form follow a lumbering up and down course. Punnett thinks these differences are so distinct that they are ^^ unlikely to be confounded by an enemy with any appreciation of color or form."
Moreover, in Ceylon at least, the distribution of the model and its mimic is very different from what is expected on the theory of mimicry. He concludes that the facts relative to their distribution '^are far from lending support to the view that the polymorphic females of P. polytes owe their origin to natural selection, in the way that the upholders of the theory of mimicry would lead us to suppose."
After considering the difficulties that the theory of mimicry has to contend with, Punnett points out that dimorphic and polymorphic species are not uncommon in butterflies, and that in many of these cases there can be httle or no question of mimicry having anything
Fig. 65. — Papilio turnus ; female (above) and male (below), and the variety P. turnus glaucus (above, right) which appears only in the female.
to do with the matter. It is well known that in Lepidoptera the modified form commonly belongs to the female sex. In one case (Abraxas grossulariata) it is known that the aberrant female type appears sporadically, as a sport, and follows Mendel's law of segregation. Punnett shows how the recurrence of the single type of male and the three types of females of polytes may also be accounted for by the recognised methods of Mendelian inheritance. He points out that by the assumption that these types have suddenly appeared as mutants many of the difficulties of the older theories are avoided, and that such an assumption is in harmony with an ever increasing body of evidence concerning variation and heredity. On this view natural selection" plays no part in the formation of these polymorphic forms," nor does sexual selection. The absence of transitional forms is explicable on this
Fig. 66. — Colias philodice, showing two female forms above and one male form below.
view, and unaccountable on the other theory. In fact polymorphic forms, if they appear, would be expected to persist if they are not harmful to the species.
We have in this country several species of butterflies in which polymorphism exists. In the north the species Papilio turnus (Fig. 65) is alike iji the male and in the female. But in the south two types of females exist — one like the male and the other ablack type.
In the Eastern States there is a butterfly, Colias philodice, in which two types of female exist (Fig. 66). Gerould has studied the mode of inheritance of these two types and finds that they conform to a scheme in which the two females differ by a single factor. The evidence is strongly in favor of the view that one of these forms has arisen as a mutation. There is no need to suppose that sexual selection has had anything to do with its origin, and no evidence that it owes its existence to mimicry of any other species.
Finally, I should like to speak of a case that has come under my own observation. One of the mutants that appeared in a culture of drosophila had a new eye color that was called eosin. In the female the eye is much deeper in color than in the male. The race maintains itself as a bicolor type without any selection.
Conclusions
In conclusion let me try to bring together the main considerations that seem to me to throw serious doubts on Darwin's theory of sexual selection.
First. Its fundamental assumption that the evolution of these characters has come about through the will," ^'choice," or selection of the female is questionable, because of want of evidence to show that the females make their choice of mates on this basis. There is also some positive evidence to show that other conditions than selection of the more ornamented individual (because he is the more ornamental) are responsible for the mating.
Second. We have come to have a different conception of what selection can do than the sliding scale assumption that has been current, at least by impUcation, in much of the post-Darwinian writings.
Third. Recent advances in the study of variations have given us a new point of view concerning the nature of variation and the origin of variations. If we are justified in applying this new view to secondary sexual characters, the problem appears greatly simplified.
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Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.
Cite this page: Hill, M.A. (2024, April 19) Embryology Heredity and Sex (1913) 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Heredity_and_Sex_(1913)_4
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