504 



THE THEORY OF EVOLUTION 



may be cenogenetic, or adaptations to the present manner of devel- 

 opment. 



To cite a familiar example, the embryo of a fish (c/. Fig. 219, 

 p. 421) develops gill slits and gills with blood vessels and a two- 

 chambered heart, which persist in the adult. The tadpole of a frog 

 develops a similar organization, which is fish-like in the arrange- 

 ment of gills and blood vessels and in the two-chambered condition 

 of the heart. But this structure is completely reorganized at 

 metamorphosis when the tadpole changes into an air-breathing 

 iinimal with the lungs and three-chambered heart of the frog. In 



Fig. 267. — Au ancient member of the shad family, Diplomystus, from the 



fishbed at Green River, Wyo. 



(From Lucas, "Animals of the Past," by courtesy American Museum of Natural History.) 



reptiles, birds, and mammals, the embryo also develops gill slits 

 (c/. Fig. 287, p. 531) and a circulatory system that is fish-like, along 

 with a two-chambered heart (cf. Fig. 226, p. 430), only to have 

 these structures reconstructed in later stages to form a different 

 adult organization. These stages of development in vertebrate 

 embryos are as well known as any facts in the anatomy of the 

 adult animal, and may be seen by any student of Embryology. 



Such stages might be expected in fishes, and perhaps in the 

 amphibians, which develop in water; but it is surprising that they 

 should occur in reptiles, birds, and mammals. In view of the 

 effectiveness of adjustment in the structure and functions of ani- 

 mals, it seems a wasteful process for an organism to develop by 

 such a round-about course. When, however, this course of 



