io8 THE ANIMALS AND MAN 



semblances and differences, but later when naturalists began 

 to dissect animals and to get acquainted with the structure 

 of the whole body, the differences and likenesses of inner 

 parts, such as the skeleton and the organs of circulation 

 and respiration, were taken into account. At the present 

 time and ever since the theory of descent began to be accepted 

 by naturalists (and there is practically no one who does 

 not now accept it), the classification of animals, while still 

 largely based on resemblances and differences among them, 

 tells more than the simple fact that animals of the same 

 group resemble each other in certain structural characters. 

 It means that the members of a group are related to each 

 other by descent, that is, genealogically. They are all the 

 descendants of a common ancestor; they are all sprung 

 from a common stock. And this added meaning of classifica- 

 tion explains the older meaning; it explains why the animals 

 are alike. The members of a group resemble each other in 

 structure because they are actually blood relations. But as 

 their common ancestor lived ages ago, we can learn the 

 history of this descent, and find out these blood-relationships 

 among animals only by the study of forms existing now, 

 or through the fragmentary remains of extinct animals pre- 

 served in the rocks as fossils. As a matter of fact we usually 

 learn of the existence of this actual blood-relationship, or 

 the fact of common ancestry among animals, by studying 

 their structure and finding out the resemblances and dif- 

 ferences among them. If much alike we believe them 

 closely related; if less alike we believe them less closely 

 related, and so on. So after all, though the present-day 

 classification means something more, means a great deal 

 more, in fact, than the classification of the earlier naturalists 

 it is still largely based on and determined by resemblances 

 and differences just as was the old classification. Some- 

 times the fossil remains of ancient animals tell us much 

 about the ancestry and descent of existing forms. For 



