EVOLUTION 



293 



limb, such as that represented by the forelimb of Amphibia. 

 In the case of vertebrate limbs and also other so-called homol- 

 ogous structures, the facts of comparative anatomy do not, of 

 course, prove evolutionary relationship any more, perhaps, than 

 a collection of automobiles including all types from the earliest 

 one-cylinder motor to the latest multicylinder type proves that 

 there is a genetic relationship between them. However, in 

 the case of biology there is another line of evidence from embry- 



A B C 



Fig. 172. — Skeleton of the right forelimb of several vertebrates to show the 

 fundamental similarity of structure. A, bird (raven); B, bat; C, whale; D, 

 ox; E, horse; F, man. c, carpals; cm, carpometatarsal; h, humerus; m, meta- 

 carpals; r, radius; u, ulna; I to v, digits. (Modified from Scott, The Theory of 

 Evolution, copyright, The Macmillan Company. By permission.) 



ology which warrants an evolutionary interpretation of facts of 

 comparative anatomy and which justifies the use of the term 

 homology. 



Development. — The law of biogenesis holds that an animal 

 recapitulates to a certain extent the history of its race. Actually 

 ontogeny presents such a mixture of old (palingenetic) and new 

 (cenogenetic) characters that the facts of development in any 

 given case must be interpreted with proper care. Nevertheless 

 there are cases where a knowledge of embryogeny has been useful 

 in determining the relationship between two or more animals. 

 Thus the subphylum Tunicata of the phylum Chordata includes 



