348 



PRINCIPLES OF ANIMAL BIOLOGY 



Many facts of biology are easily explained as due to evolution, but are 

 almost inexplicable on any other assumption. These facts come from 

 various fields of which comparative anatomy is one of the richest. 



Comparative Anatomy. — Thus, the similarity of structure in dif- 

 ferent animals, as revealed by the study of comparative anatomy, is 

 precisely what would be expected if evolution occurred, but would be 

 without meaning in the absence of evolution. The discovery of similar 



< S C/77 > 



Fig. 239. — Mutation in CEnothera involving the rosettes, or young plants. Below 

 (8 and 9) CEnofhera pratincola; above (3 and 4) CEnothera pratincola mutation iiummularia, 

 a mutant of the preceding form. {Photo by Professor H. H. Bartlett.) 



bones in the limbs of practically all vertebrates, though with modifications 

 in each, can hardly mean anything else than that these animals have de- 

 scended from common ancestors. The similarity of the labyrinth of 

 the inner ear in various vertebrate animals (Fig. 240) is explicable on the 

 same hypothesis. That is why homology is regarded as a certain sign 

 of relationship. The differences found in homologous structures are 



