145 



words, it tries to connect by some process of deri- 

 vative descent the higher with the lower, and the 

 highest with that which stands next beneath it. 



In working out this inquiry we must, as in other 

 cases, believe iuthefixity of nature's methods, and apply 

 our knowledge of the present to the explanation of the 

 past. We know that life is influenced by the physical 

 conditions by which it is surrounded j we also know 

 that under these influences some species succumb, 

 while others, more elastic ia their nature, become 

 slightly modified, and endure ; and we further know 

 that such variations are perpetuated by hereditary 

 transmission. If, then, the principle of variation or 

 adaptive modification be admitted — no matter how 

 infinitesimal the variations may be within a given 

 time — it must, in the long run, be capable of producing 

 the most extensive results, and what are now regarded 

 as varieties may pass into species, and species in 

 process of time assume the rank of generic dis- 

 tinctions. We are aware it is argued on the other 

 side that, though variation is incessantly taking place, 

 yet there is a limit to its extent, and that in time the 

 varieties either die out or return to the typical species. 

 As perturbations in the planetary system have a limit, 

 so it is contended variation in life-forms has its limit, 

 and does not and cannot go on indefinitely. Plausible 

 as this may seem, it is, however, the merest assump- 



