THE EVOLUTION OF ANIMAL LIFE 



Vestigial structures are of interest in the study of homology. These are 

 structures which correspond in plan and in position to functional parts of 

 related animals, but which are much reduced in structure and may be 

 apparently functionless. Examples include the splint bones in the limbs of 

 modern horses, the vestigial second and fourth digits; the tiny remnants of the 

 hind limbs in pythons, representing structures which have entirely disap- 

 peared in most snakes; and the vestigial hind limbs of whales and other 

 aquatic mammals. Vestigial structures are numerous among the more highly 

 specialized members of any phylum. 



We are thus faced with the problem of accounting for these facts of 

 anatomy, in terms of the history of animal life. In pre-evolutionary inter- 

 pretations, each species of animal, though created separately, was thought 

 to have been formed in accordance with an ideal type, which explained the 

 existence of homologous structures. This was the belief of Louis Agassiz 

 (1807-1873), who, although a naturalist of great ability, was never able to 

 adjust his thinking to the concept of evolution as set forth by Darwin in 

 1859. The idea of special creation after ideal forms is, of course, a theoretical 

 possibility; to substantiate it, there should be objective evidence that animals 

 originated in their present form and have never changed. As we have con- 

 cluded from our survey of the fossil record, however, animals evidently did 

 not originate in their present form but have changed remarkably during 

 geologic time. The evolutionary explanation of the anatomical resemblances 

 between animals states that all the members of a group, such as the verte- 

 brates, have inherited a similar plan of organization from the common 

 ancestors of all vertebrates. Each subdivision and species has been modified 

 in particular ways in relation to its way of life; but all remain fundamentally 

 similar, because they have never lost the underlying body plan that char- 

 acterized their ancestors. Because whales and all other mammals had a com- 

 mon ancestry at a remote period, their fore limbs and many other parts retain 

 certain similarities at present. Mammals resemble other vertebrates for 

 similar reasons. Vestigial structures remain because they are a part of a 

 persistent plan and have not been entirely lost, although thev may be no longer 

 functional. The comparative anatomy of adult organisms furnishes very tell- 

 ing arguments for the occurrence of organic evolution. 



Comparative Embryology. Almost everyone who is at all familiar with 

 the evidence for organic evolution has heard the statement that the individual 

 in its development repeats the developmental history of the race. This gen- 

 eral idea often takes the form of the aphorism, "Ontogeny recapitulates 

 phylogeny." There is probably no statement in the whole field of biology 

 that is more widely misunderstood. The Recapitulation Theory, as it has been 

 called, maintains that certain developmental stages or structures are related 

 to ancestral conditions; it is recognized that others may be adaptations to the 

 present manner of development. 



To cite a familiar example, the embryo of a fish develops gill slits, gills with 

 specific blood supply, and a two-chambered heart; all these features persist 



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