Page Six 



EVOLUTION 



November. 1928 



The Proofs of Evolution 



By Hexshaw Ward, 



{T/iis is tlie last of a series of three articles. The proofs 

 described in the previous articles are: I. The specialists are unan- 

 imous. II. If^hat breeditiff proves. III. What the rocks prove, 

 ir. If hat t/coiiraphy proves.) 



V. What Stkuctures Prove 



THE breeders have proved that plants and animals can be 

 altered — gradually, through a series of generations — to forms 

 that are very different from the original ancestor. For instance: 

 a small, single daisy growing ^vild in China has been made to 

 evolve into a double chrysanthemum twenty inches in diameter ; 

 heavy draft horses and race horses have been evolved from a 

 common ancestor; the tiny, feeble, hairless dog of Mexico and 

 the fierce, stiff-haired police-dog evolved from a common an- 

 cestry. These evolutions are facts in human history, just as the 

 evolution of certain shells is a fact in the rocks. The two lines 

 of proof — from breeders' records and from geology — fit together. 



And both these lines of proof agree with a third line — the 

 facts recorded in the skeletons of animals. Suppose that we had 

 before us a set of a hundred photographs of the changes that 

 took place through a hundred generations in the breeding of a 

 small, hairless dog. The first photograph would show a litter 

 of five pups, and an arrow would point to one that was selected 

 because it was slightly smaller and less hairy than the other four. 

 The second photograph would show the son of this pup, slightly 

 different from its father. The third photograph would show 

 slightly more variation in the third generation, and so on to 

 the final product in the hundredth generation. At no point 

 would there be a greater difference between father and son than 

 we are used to seeing in any family of animals, but the differ- 

 ence between the first father and his remote descendant would 

 be amazing. 



Now suppose that a second series of photographs showed 

 another line of selection from the original litter, each generation 

 being a trifle bonier and having stiffer hair than the one before 

 it, till in the hundredth generation there was a descendant that 

 resembled a police-dog. Put the two products of evolution side 

 by side, and it seems a miracle that they descended from a 

 common ancestor. Yet any producer of 

 domesticated plants or animals could tell 

 true stories from the history of breeding 

 that are stranger than this imaginary case. 

 The differences between the feeble pet 

 dog and the fierce police-dog — great as 

 they seem — are all within limits. Each 

 has four legs, two eyes, one nose and one 

 tail. Each has the same number of toes, 

 the same bones in its ears, the same num- 

 ber of sections in its backbone, the same 

 kinds of teeth in its jaws. Therefore a 

 special student of anatomy, if he saw the 

 two skeletons mounted in a museum, would 

 know absolutely that there is a close family 

 resemblance between them. He would 

 know that they are more closely related 

 to each other than they are to a creature 

 with a different sort of spine or a different 

 outfit of teeth. 



This one simple case illustrates the way 

 in which anatomists can trace the line of 

 descent in different kinds of animals, by 

 comparing the structures of animals now 

 living and by comparing the fossils of 

 animals. If we see a seal swimming, we 

 may think it is a sort of fish ; but in its 

 flippers we find the same five-finger struc- 

 ture that is in all warm-blooded animals 

 and that is not in any fish; we find the 



^ame three leg-bones that are in man. We might suppose that 

 a whale is a sort of fish; but it is a warm-blooded animal that 

 suckles its young; its flippers contain the same five fingers and 

 three arm-bones that we have ; buried in the flesh of its tail are 

 the remnants of leg-bones and toes. Since its structure is so 

 similar to ours, we know that it is very much more closely 

 related to us than any fish is. 



The anatomists have discovered a fossil record of the evolu- 

 tion of the horse; it stretches back millions of years, through 

 forms that get smaller and smaller, through forms that had two 

 toes and four toes, back to an ancestor only a foot high. Almost 

 as complete a series of fossils shows the evolution of the elephant 

 from a snouted creature only three feet high. 



These structures that are found in the rocks, and the struc- 

 tures of the animals that live today, are a record of alterations 

 that have taken place in the course of thousands and millions 

 of years. 



VI. What Embryos Prove 



If the proofs in skeletons and islands and fossils all point 

 strongly to the idea that plants and animals have come to their 

 present forms by evolution, their combined evidence is very 

 strong. Their strength is doubled when it is put with the proof 

 that embryos furnish. 



Every animal begins its life as a single cell that is too small 

 to be seen with the naked eye. Every mouse and tiger and cow 

 and man begins its career in a sac so small that the most, power- 

 ful microscope cannot distinguish one kind from the other. This 

 cell divides into two cells, each of these into two others, and 

 so on until millions and billions of them have shaped them- 

 selves into an animal that lives inside the mother. This earliest 

 stage of the individual's life is called an embryo. When the 

 embryo of a chick is a few days old in the egg-shell, it does 

 not look like a fowl ; it looks like a gill-breathing fish. Also 

 the embryo of a calf or of a rabbit looks like a fish. Also the 

 embryo of a man looks like a fish. The most learned professor 

 in a medical school could not tell, in the earliest stages, whether 

 an embryo is to develop into a rooster or a man. 



Every embryo of a warm-blooded animal goes through a 

 development — in its own short career of a few weeks or months 



Parallelism in the embryonic developments of various animals. 



-From Hird ajtfr Haeckel. 



