preserve a number of early acting mutations 

 that have survival significance during the 

 larval stages. Later in development many of 

 these special larval adaptations are lost, for 

 during metamorphosis, the structures of the 

 adult are derived from less specialized parts 

 of the larva. In short, differences between 

 related species may appear at any stage of 

 development — embryonic, larval, or adult — 

 although it is undeniably true that genetic 

 relationships are apt to be plainer in the 

 earlier stages of development. 



Higher animals, due to the greater com- 

 plexity of their mature structure, tend to 

 differ more drastically from their respective 

 embryos than do their lower, or simpler, rela- 

 tives. For this reason, the embryonic stages 

 of higher animals are apt to exhibit striking 

 resemblances to the adult stages of lower 

 species. For instance, the embryos of all ver- 

 tebrates develop gill slits in the pharynx 

 (Fig. 29-2); and in lower vertebrates (fish) 

 these gill slits remain as functional respira- 

 tory channels in the adult species. But in the 

 embryos of higher air-breathing vertebrates 

 (Fig. 29-3), the gill slits are closed, except for 

 the first pair, which becomes modified to 

 form the Eustachian tubes (p. 283). Appar- 

 ently the genes responsible for the develop- 

 ment of gill slits in the ancestral vertebrate 

 stock have persisted, while other genes, which 

 decree the closing of these channels, exert 

 their developmental effects later in the em- 

 bryonic period. In a sense, therefore, gill slits 

 are to be regarded as embryonic vestigial 

 structures. 



Biologists of the eighteenth and nine- 

 teenth centuries were deeply impressed by 

 the resemblance of early embryonic stages of 

 higher animals to the adult stages of lower 

 members of the same class or phylum. In fact 

 these observations formed the basis of the 

 theory of recapitulation, which was accepted 

 very widely. Briefly, the recapitulation theory 

 held that every species, in passing through its 

 embryonic stages, repeats the evolutionary 

 stages by which the species has reached its 

 present status. Without question the recapi- 



The Consequences of Evolution - 557 



tulation theory served a useful purpose in 

 emphasizing the general similarity between 

 embryonic development and evolutionary de- 

 velopment. All multicellular animals do, in 

 fact, revert to the unicellular condition at 

 conception; and the blastula, gastrula, and 

 other generalized stages of embryonic devel- 

 opment do tend to resemble the early steps 

 of evolutionary development. But the theory 

 cannot sustain a very detailed analysis, as 

 might be expected in the light of our mod- 

 ern knowledge of genetics and development. 

 So long as a given set of genes survives in any 

 kind of organism, these ancient genes will 

 continue to preserve the ancient character- 

 istics of the stock, although newer genes may 

 modify and obscure these old effects at any 

 stage of development. Moreover, some genes 

 in every stock seem to be so crucial in em- 

 bryonic development that few, if any, muta- 

 tions in these genes are tolerable to the 

 organism. Consequently such genes are pre- 

 served in the course of natural selection and 

 are responsible for the recapitulation of cer- 

 tain age-old characteristics in every species. 



BIOCHEMICAL RELATIONSHIPS AMONG 

 ORGANISMS 



Genetic relationships between different 

 organisms are clearly indicated not only by 

 their structural and developmental charac- 

 teristics, but also by their chemical composi- 

 tion — particularly with reference to their 

 protein components. The proteins extracted 

 from corresponding tissues of closely related 

 animals tend to be very similar, or sometimes 

 identical; whereas more distantly related 

 species show much greater differences in this 

 respect. Take, for example, the blood pro- 

 teins of vertebrate animals, which have been 

 studied very extensively in recent years. The 

 antigenic reactions (p. 335) of bloods from 

 the various races of man prove to be prac- 

 tically identical, and almost identical with 

 the antigenic reactions of bloods from the 

 anthropoid apes. The bloods of Old World 

 monkeys come next in their relationship to 



