Book - Evolution and Genetics 2
|Embryology - 7 May 2021 Expand to Translate|
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Chapter 2 The Four Great Historical Speculations
Looking' backward over the history of the evolution theory we recognize that during the hundred and odd years that have elapsed since Buff on, there have been four main lines of speculation concerning evolution. We might call them the four great cosmogonies or the four modern epics of evolution.
GEOFFROY ST. HILAIRE
About the beginning of the last century Geoffrey St. Hilaire, protege, and in some respects a disciple, of Buffon, was interested as to how living species are related to the animals and plants that had preceded them. He was familiar with the kind of change that takes place in the embryo if it is put into new or changed surroundings, and from this knowledge he concluded that as the surface of the earth slowly changed — as the carbon dioxide contents in the air altered — as land appeared, and as marine animals left the water to inhabit it, they or their embryos responded to the new conditions and those that responded favorably gave rise to new creations. As the environment changed the fauna and flora changed — change for change. Here we have a picture of progressive evolution that carries with it an idea of mechanical necessity. If there is anything mystical or even improbable in St. Hilaire's argument it does not appear on the surface ; for he did not assume that the response to the new environment was always a favorable one or, as we say, an adaptation. He expressly stated that the response was unfavorable the individual or the race died out. He assumed that sometimes the change might be favorable, i.e., that certain species, entire groups, would respond in a direction favorable to their existence in a new environment and these would come to inherit the earth. In this sense he anticipated certain phases of the natural selection theory of Darwin, but only in part ; for his picture is not one of strife within and without the species, but rather the escape of the species from the old into a new world.
If, then, we recognize the intimate bond in chemical constitution of living things and of the world in which they develop, what is there improbable in St. Hilaire's hypothesis? Why, in a word, is not more credit given to St. Hilaire in modern evolutionary thought ? The reasons are to be found, I think, first, in that the evidence to which he appealed was meagre and inconclusive; and, second, in that much of his special evidence does not seem to us to be applicable. For example the monstrous forms that development often assumes in a strange environment, and with which every embryologist is only too familiar, rarely if ever furnish combinations, as he supposed, that are capable of living. On the contrary, they lead rather to the final catastrophe of the organism. And lastly, St. Hilaire's appeal to sudden and great transformations, such as crocodile's egg hatching into a bird, has exposed his view to too easy ridicule.
But when all is said, St. Hilaire's conception of evolution contains elements that form the background of our thinking today, for taken broadly, the interaction between the organism and its environment was a mechanistic conception of evolution even though the details of the theory were inadequate to establish his contention.
In our own time the French metaphysician Bergson in his Evolution Cr^eat^iee has proposed in mystical form a thought that has at least a superficial resemblance to St. Hilaire's conception. The response of living things is precise, exact, yet not mechanical in the sense at least in which we usually employ the word mechanical. For Bergson claims that the one chief feature of living material is that it responds favorably to the situation in which it finds itself, at least so far as lies within the possible physical limitations of its organization. Evolution has followed no preordained plan; it has had no creator; it has brought about its own creation by responding adaptively to each situation as it arose.
But note: the man of science believes that the organism responds today as it does, because at present it has a chemical and physical constitution that gives this response. We find a specific chemical composition and generally a specific physical structure already existing. We have no reason to suppose that such particular reactions would take place until a specific chemical configuration had been acquired. Where did this constitution come from? This is the question that the scientist asks himself. I suj^pose Bergson would have to reply that it came into existence at the moment that the first specific stimulus was applied. But if this is the answer we have passed at once from the realm of observation to the realm of fancy — to a realm that is foreign to our experience ; for such a view assumes that chemical and physical reactions are guided by the needs of the organism when the reactions take place inside living beings.
Use and Disuse
FROM LAMARCK TO WEISMANN
The second of the four great historical explanations appeals to a change not immediately connected with the outer world, but to one within the organism itself.
Practice makes perfect is a familiar adage. Not only in human affairs do we find that a part through use becomes a better tool for performing its task, and through disuse degenerates; but in the field of animal behavior we find that many of the most essential types of behavior have been learned throughly repeated associations formed by contact with the outside.
It was not so long ago that we were taught that the instincts of animals are the inherited experience of their ancestors — lapsed intelligence was the current phrase.
Lamarck's name is always associated with the application of the theory of the inheritance of acquired characters. Darwin fully endorsed this view and made use of it as an explanation in all of his writings about animals. Today the theory has few followers amongst trained investigators, but it still has a popular vogue that is widespread.
To Weismann more than to any other single individual should be ascribed the disfavor into which this view has fallen. In a series of brilliant essays he laid bare the inadequacy of the supposed evidence on which the inheritance of acquired characters rested. Your neighbor's cat, for instance, has a short tail, and it is said that it had its tail pinched off by a closing door. In its litter of kittens one or more is found without a tail. Your neighbor believes that here is a case of cause and effect. He may even have known that the mother and grandmother of the cat had natural tails. But it has been found that short tail is a dominant character; therefore, until Ave know who was the father of the short-tailed kittens the accident to its mother and the normal condition of her maternal ancestry are not to the point.
Weismann appealed to common sense. He made few experiments to disprove Lamarck's hypothesis. True, he cut off the tails of some mice for a few generations but got no tailless offspring and while he gives no exact measurements with coefficients of error, he did not observe that the tails of the descendants had shortened one whit. The combs of fighting cocks and the tails of certain breeds of sheep have been cropped for many generations and the practice continues today, because sheep's tails are still long, and cocks still grow combs.
The Unfolding Principle
I have ventured to put down as one of the four great historical explanations, under the heading of the unfolding principle, a conception that has taken protean forms. At one extreme it is little more than a mystic sentiment to the effect that evolution is the result of an inner driving force or principle which goes under many names such as Bildungstrieb, nisus formativus, vital force, and orthogenesis. Evolutionary thought is replete with variants of this idea, often naively expressed, sometimes unconsciously implied. Evolution once meant, in fact, an unfolding of what pre-existed in the egg, and the term still carries with it something of its original significance.
Nageli's speculation may be taken as a typical case. Nageli thought that there exists in living material an innate power to grow and expand. He vehemently protested that he meant only a mechanical principle but, as he failed to refer such a principle to any properties of matter known to physicists and chemists, his view seems still a mysterious affirmation as difficult to understand as the facts themselves which it purports to explain.
Nageli compared the process of evolution to the growth of a tree, whose ultimate twigs represent the living world of species. Natural selection plays only the role of the gardener who prunes the tree into this or that shape but who has himself produced nothing. As an imaginative figure of speech Nageli's comparison of the tree might even today seem to hold if we substituted propagation and variation for "growth," but although we know so little about what causes variation there is no reason for supposing it to be due to an inner vague impulse.
In his recent presidential address before the British Association, Bateson has inverted this idea. I suspect that his effort was intended as little more than a tour de force. He claims for it no more than that it is a possible line of speculation. Perhaps he thought the time had come to give a shock to our too confident views concerning evolution. Be this as it may, he has invented a striking paradox. Evolution has taken place through the steady loss of inhibiting factors. Living matter was stopped down, so to speak, at the beginning of the world. As the stops are lost, new things emerge. The germinal material has changed only in that it has become simpler.
Of the four great historical speculations about evolution, the doctrine of Natural Selection of Darwin and Wallace has met with the most widespread acceptance. Later the theory will be examined more critically. Here only its broadest aspects will be considered.
Darwin appealed to chance variations as supplying evolution with the material on which natural selection works. If we accept, for the moment, this statement as the cardinal doctrine of natural selection it may appear that evolution is due, ( 1 ) not to an orderly response of the organism to its environment, (2) not in the main to the adjustment of the animal through the use or disuse of its parts, (3) not to any innate non-physical principle of living material itself, and (4) above all not to purpose either from within or from without. Darwin made quite clear what he meant by chance. By chance he did not mean that the variations were not causal. On the contrary he taught that in science we mean by chance only that the particular combination of causes that bring about a variation is not known.
They are accidents, it is true, but they are causal accidents.
In his famous book on Animals and Plants under Domestication, Darwin dwells at great length on the nature of the conditions that bring about variations. If some of his views seem to us today at times vague, at times problematical, and often without a secure basis, nevertheless we find, in every instance, that Darwin was searching for the physical causes of variation. He brought, in consequence, conviction to many minds that there are abundant indications, even if certain proof is lacking, that the causes of variation are to be found in natural processes.
Today the belief that evolution takes place by means of natural processes is generally accepted. It did not seem probable that we should ever again have to renew the old contest between evolution and special creation.
But this is not enough. We can never remain satisfied with a negative conclusion of this kind. We must find out what natural causes bring about variations in animals and plants; and we must also find out what kinds of variations are inherited, and how they are inherited. If the circumstantial evidence for organic evolution, furnished by comparative anatomy, embryology and palaeontology is cogent, we should expect to observe evolution going on at the present time, i.e., we should be able to observe the occurrence of variations and their transmission. This has actually been done by the geneticist. Certain kinds of new characters have been seen to arise by a process called mutation and their inheritance is now known to follow Mendel's laws.
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