CLASSIFICATION A TREE-LIKE STRUCTURE. 63 



nized, it must be acknowledged that different mor- 

 phologists have not reached the same conclusions as 

 to these homologies in all cases. 



But it is undoubtedly a fact that to a certain ex- 

 tent homologies can be traced between animals as 

 widely separated as the so-called types. It is 

 further a significant fact that these homologies are 

 most noticeable when the lower members of the dif- 

 ferent types are compared with each other ; in other 

 words, the types approach each other at the bottom. 

 The lowest vertebrates approach much more closely 

 to the invertebrates than do the higher members of 

 this group ; so close indeed that in some cases low 

 vertebrates have been classed with one or another 

 of the invertebrate classes. All of this would of 

 course naturally follow from the descent theory ; 

 for the lower members are nearer to the supposed 

 common ancestor. And this theory would lead us 

 to expect that there might be some animals inter- 

 mediate between the great types, which represent 

 their common starting-point. Very careful search 

 after such forms has been made, and certain small 

 circumscribed groups have been found, consisting 

 perhaps of only a single species, which show so 

 many general likenesses that they do not seem to 

 belong anywhere, but have been changed about 

 from place to place, now with one class of animals 

 and now with another, until it has been assumed by 

 the advocates of the evolution hypothesis that they 

 form connecting links between the great types. 

 This point has been strongly emphasized by some 

 naturalists, and would be highly important if true; 



