66 ELEMENTARY ZOOLOGY 



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 ac- 

 count. 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 classifi- 

 cation 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 mem- 

 bers 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 classification 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 

 preserved 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 differences 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 means, it is largely based on 

 and determined by resemblances and differences just as 

 was the old classification. Sometimes the fossil remains 

 of ancient animals tell us much about the ancestry and 

 descent of existing forms. For example, the present-day 



