PRINCIPLES OF TAXONOMY 



253 



Structures that arise in the same way in evolution are said to be 

 homologous with one another or to exhibit homology. Homology means 

 similarity of origin in evolution. Unfortunately for the taxonomist the 

 early evolution of the structures on which his classification is based took 

 place in many instances millions of years ago. How can he ascertain, 

 under these circumstances, whether the evolution of structures in two 

 animals was similar or not? The answer to this question must usually 

 be arrived at indirectly. 



Homology Judged from Adult Structure. — The most reliable means 

 of judging of similarity of evolution in two groups would be fossil mem- 

 bers of those groups, if fossils could be obtained in sufficient numbers to 

 establish a fairly complete history extending far back into their ancestry. 

 Some such histories are given in Chap. 22. In most families of animals, 



WALKING 



FLYING 



SWIMMING DIGGING HANDLING 



salamander 



crocodile 



carpals 

 meiacarpals 



phalanges 

 mole 



man 



bird bat whale 



Fig. 208. — Homology in the bones of the fore Umbs of vertebrates. Numbers 1-5 refer to 

 digits, from thumb to Uttle finger. {From Storer, "General Zoology.") 



however, good fossil series are wanting, and the taxonomist must rely 

 on what can be discovered from the living animals of today. In clear 

 cases adult structure is sufficient, but only where many features are 

 alike in the animals in question. A classical case of homology, judg- 

 ment of which could safely rest on adult structure alone, is that which 

 exists among the forelimbs of vertebrate animals (arms, wings, forefeet, 

 etc.. Fig. 208). Although the external forms of these forelimbs differ 

 greatly in birds, seals, horses, whales, bats, and man, their skeletons are 

 found to correspond very closely, bone for bone, at most points. It is 

 believed that so many similarities could not be the result of accident or of 

 convergence from originally distinct sources and that the likenesses are 

 a sign of similar evolutionary origins. The nervous systems of vertebrate 

 animals are equally good examples. The parts of the brain in fishes, 

 amphibia, reptiles, birds, and mammals have a very obvious correspond- 

 ence, and the origin and distribution of the cranial nerves are very similar 

 in all of them. It is scarcely conceivable that these nervous systems could 



