Book - Evolution and Genetics 1

From Embryology

Morgan (1925) Evolution and Genetics: 1 Different Kinds of Evolution | 2 Four Great Historical Speculations | 3 Evidence for Organic Evolution | 4 Materials of Evolution | 5 Mendel's Two Laws of Heredity | 6 Chromosomes and Mendel’s Two Laws | 7 Linkage Groups and the Chromosomes | 8 Sex-Linked Inheritance | 9 Crossing-over | 10 Natural Selection and Evolution | 11 Origin of Species by Natural Selection | 12 Non-Inheritance of Acquired Characters | 13 Human Inheritance | Figures

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter I Different Kinds of Evolution

We use the word evolution in many ways — to include many different kinds of changes. There is hardly any other scientific term that is used so carelessly — to imply so much, to mean so little.


We speak of the evolution of the stars, of the evolution of the horse, of the evolution of the steam engine, as though they were all part of the same process. What have they in common? Only this, that each concerns itself with the history of something. When the astronomer thinks of the evolution of the earth, the moon, the sun and the stars, he has a picture of diffuse matter that has slowly condensed. With condensation came heat; with heat, action and reaction within the mass until the chemical substances that we know today were produced. This is the nebular hypothesis of the astronomer. The astronomer explains, or tries to explain, how this evolution took place, by an appeal to the physical processes that have been worked out in the laboratory, processes which he thinks have existed through all the eons during which this evolution was going on and which were its immediate causes.


When the biologist thinks of the evolution of animals and plants, a different picture presents itself. He thinks of series of animals that have lived in the past (fig. 1) whose bones and shells have been preserved in the rocks (fig. 2) . He thinks of these animals as having in the past given birth, through an unbroken succession of individuals, to the living inhabitants of the earth today. He thinks that some of the simpler types of the past have in part changed over into the more complex forms of the present time.

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Fig. 1. Outline of horses of different geological periods, showing their relative sizes and the decrease in the number of toes. (After Lull.)

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Fig. 2. Forefeet of horses, showing the progressive loss of the lateral toes. (After Marsh, from Lull.)

He is thinking as the historian thinks, but he runs the risk of thinking that he is explaining evolution when he is only describing it.


A third kind of evolution is one for which man himself is responsible, in the sense that he has brought it about, often with a definite end in view.


His mind has worked slowly from stage to stage. We can often trace the history of the stages through which his creative processes have passed. The evolution of the steam-boat, the steam engine, paintings, clothing, instruments of agriculture, of manufacture, or of warfare (fig, 3) illustrates the history of human progress. There is an obvious and striking similarity between the evolution of man's inventions and the evolution of the shells of molluscs and of the bones of mammals, vet in neither case does a knowledge of the order in which these things arose explain them. If we appeal to the psychologist he will probably tell us that human inventions are either the result of happy accidents, that have led to an unforeseen, but discovered use; or else the use of the invention was foreseen. It is to the latter process more especially that the idea of purpose is applied. When we come to review the four great lines of evolutionary thought we shall see that this human idea of purpose recurs in many forms, suggesting that man has often tried to explain how organic evolution has taken place by an appeal to the method which he believes he makes use of himself in the inorganic world.

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Fig. 3. Evolution of helmets. (After Dean.)

What, I repeat, has the evolution of the stars in common with the evolution of the horse, and what have these in common with the evolution of human inventions? Clearly no more than that from a simple beginning through a series of changes something more complex, or at least different, has come into being. To lump all these kinds of changes into one and call them evolution is only to assert that you believe in consecutive series of events (which is history) causally connected (which is science). It is the aim of science to find out specifically what kinds of causes were at work when the stars evolved in the sky, when the horse evolved on the earth, and the steam engine was evolved from the mind of man.



Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Morgan (1925) Evolution and Genetics: 1 Different Kinds of Evolution | 2 Four Great Historical Speculations | 3 Evidence for Organic Evolution | 4 Materials of Evolution | 5 Mendel's Two Laws of Heredity | 6 Chromosomes and Mendel’s Two Laws | 7 Linkage Groups and the Chromosomes | 8 Sex-Linked Inheritance | 9 Crossing-over | 10 Natural Selection and Evolution | 11 Origin of Species by Natural Selection | 12 Non-Inheritance of Acquired Characters | 13 Human Inheritance | Figures