GENERAL ZOOLOGY 



in the adult fish. The development of a frog tadpole includes the formation 

 of a similar organization, fish-like in the arrangement of gills and blood 

 vessels and in the two-chambered condition of the heart. But these structures 

 are completely reorganized during later development, which culminates in the 

 metamorphosis of the tadpole into an air-breathing adult with lungs and a 

 three-chambered heart. Embryonic development in reptiles, birds, and mam- 

 mals produces rudimentary gill slits and a fish-like circulatory system, which 

 are reconstructed in later stages to form a different adult organization. These 

 stages in the development of vertebrate embryos are as well known as the 

 facts of adult anatomy and can be observed by any student of embryology. 

 It is not surprising that the embryos of fishes, and even of amphibians, 

 should develop gill slits, gills, and associated blood vessels; these forms develop 

 in water, and the structures can be related to definite functions. But why 

 do gill slits develop in reptiles, birds, and mammals, which are terrestrial 

 organisms? Viewed in the light of an evolutionary interpretation, these 

 developmental peculiarities are understandable. If the ancestors of verte- 

 brates were originally fish-like animals, and if, as we have supposed on other 

 grounds, the amphibians represent a transition from aquatic to terrestrial 

 life, the modification of their developmental processes is a logical consequence. 

 In reptiles, birds, and mammals, which are true terrestrial animals in develop- 

 ment and as adults, the later stages have been modified, but the early stages 

 still resemble those of the ancestral fish-like forms. It is not correct to say 

 that the embryo of a reptile, bird, or mammal is a fish at any stage of its 

 development; but it may be truly said that these embryos resemble the 

 embryos of fishes at corresponding stages (cf. Fig. 5.22, p. 157). The state- 

 ment that the animal "climbs the ancestral tree" in the course of its de- 

 velopment is colorful but inaccurate. Many animals have in the course of 

 their development structures that disappear or become greatly modified in 

 later stages, and in many animals there is reason to believe that structures 

 now transitory were formerly present and functional in the adult as well as in 

 the embryonic stages. The fish embryo becomes an adult that is in many 

 ways similar to the embryo, whereas the embryo of a terrestrial vertebrate 

 advances beyond the fish-like stages of its embryogeny to develop the adult 

 characteristics of the reptile, bird, or mammal. Ontogeny, the development 

 of the individual, is not a repetition of phylogeny, the ancestral development; 

 but certain structures that are reminiscent of ancestry may be found in 

 embryos. As with adult anatomy, such facts of development are only to be 

 expected if evolution has occurred; and they are inexplicable on any 

 other grounds. 



EVIDENCE FROM PHYSIOLOGY 



All animals are alike in the basic capacities, of metabolism, responsive- 

 ness, and reproduction, despite innumerable variations in the details of these 

 processes. Therefore, we may reasonably suppose that all have descended 



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