PROGRESS 15 



stratum belong. Sometimes, however, as in many 

 groups of mammals, the gaps are few and small, the 

 seriation almost complete. In any event we have 

 here evidence which, so far as it goes, is perfectly 

 admissible for the main lines and for many of the 

 smaller branches of evolutionary descent 



Unfortunately, it does not go very far — or, we had 

 better say, it is of restricted application. By the time 

 we find well-preserved fossils in the rocks, the main 

 groups of the animal kingdom and their chief sub- 

 divisions had been already differentiated, with the 

 one important exception of the vertebrates ; while 

 time, heat, and pressure have so modified the earlier 

 strata as to destroy the fossil forefathers of insects, 

 molluscs, Crustacea, and the rest, which they must 

 have contained. 



Within the vertebrate stock, then, we can learn 

 a great deal from the semi-direct methods of paleont- 

 ology : but for the history of the other groups and 

 for their origin and interrelations, we are driven 

 back upon comparative anatomy and embryology, 

 into another field of more circumstantial evidence. 

 When, for instance, we find that the fore-limbs of 

 bat, bird, whale, horse, and man, although so different 

 in function and in detail of structure, are yet built 

 upon the same general plan, and upon a plan wholly 

 different from that of the limbs, say, of a spider or 

 an insect, we must either deny reason and say that 

 this similarity means nothing ; or assume that its 



