PROGRESS, BIOLOGICAL AND OTHER 15 



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 per- 

 fectly 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 fmd well-preserved fossils in the rocks, the 

 main groups of the animal kingdom and their chief 

 subdivisions 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 paleon- 

 tology: but for the history of the other groups and 

 for their origin and interrelations, we are driven back 

 upon comparative anatomy and embryology, into an- 

 other 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 func- 

 tion 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 similar- 

 ity means nothing; or assume that its cause is super- 

 natural, outside the province of science, that it is the 

 expression of some eternal Idea, or some plan of 



