732 GENETICS AND EVOLUTION 



The early human embryo at this stage resembles a fish embryo, with gill 

 pouches, pairs ol aortic arches, a fishlike heart with a single atrium and 

 ventricle, a primitive fish kidney, and a well differentiated tail complete 

 with muscles for wagging it (Fig. 35.6). At a slightly later stage the 

 human embryo resembles a reptdian embryo. Its gill pouches regress; 

 the bones which make up each vertebra, and which had been separate 

 as in the most primitive fishes, fuse; a new kidney, the mesonephros, 

 forms and the pronephros disappears or becomes incorporated into other 

 structures; and the atrium becomes divided into right and left chambers. 

 Still later in development the human embryo develops a mammalian, 

 four-chambered heart and a third kidney, the metanephros. During the 

 seventh month of intrauterine development the human embryo, with 

 its coat of embryonic hair and in the relative size of body and limbs, 

 resembles a baby ape more than it resembles an adult human. 



Our increasing understanding of physiological genetics provides us 

 with an explanation of the phenomenon of recapitulation. All chordates 

 have in common a certain number of genes which regulate the processes 

 of early development. But, as our ancestors evolved from fish, through 

 amphibian and reptilian stages, they accumulated mutations for new 

 characteristics but kept some of the original "fish" genes, which still 

 control early development. Later in development the genes which the 

 human shares with amphibians influence the course of development so 

 that the embryo resembles a frog embryo. Subsequently some of the genes 

 which we have in common with reptiles come into control. Only after 

 this do most of the peculiarly mammalian genes exert their influence, 

 and these are followed by the action of genes we have in common with 

 other primates. The anthropoid apes, which have the most immediate 

 ancestors in common with us, have the most genes in common with us 

 and their development is identical with ours except for some fine details. 

 A pig or rat, whose ancestors are the same as ours only up to the stage of 

 the primitive placental mammals, has fewer genes in common and has 

 developmental processes that diverge at an earlier time. In general dur- 

 ing development, the general characters that distinguish phyla and 

 classes appear before the special characters that distinguish genera and 

 finally species. Within each phylum, the higher forms pass through a 

 sequence of developmental stages which are similar to those of lower 

 forms, but achieve a different final form by adding changes at the end 

 of the original sequence and by altering certain of the earlier embryonic 

 stages they share with the lower forms. 



314. Evidence from Genetics and Cytology 



For the past several thousand years man has been selecting and 

 breeding animals and plants for his own uses, and a great many varieties, 

 adapted for different purposes, have been established. These results of 

 artificial selection provide striking models of what may be accomplished 

 by natural selection. All of our breeds of dogs have descended from one, 

 or perhaps a very few, species of wild dog or wolf, yet they vary so much 

 in color, size and body proportions that if they occurred in the wild 



