Chap. XIV. CONCLUSION. 481 



asserted that organic beings in a state of nature are 

 subject to no variation ; it cannot be proved that the 

 amount of variation in the course of long ages is a 

 limited quantity ; no clear distinction has been, or can 

 be, drawn between species and well-marked varieties. 

 It cannot be maintained that species when intercrossed 

 are invariably sterile, and varieties invariably fertile ; 

 or that sterility is a special endowment and sign of 

 creation. The belief that species were immutable pro- 

 ductions was almost unavoidable as long as the history 

 of the world was thought to be of short duration ; and 

 now that we have acquired some idea of the lapse of 

 time, we are too apt to assume, without proof, that the 

 geological record is so perfect that it would have 

 afforded us plain evidence of the mutation of species, 

 if they had undergone mutation. 



But the chief cause of our natural unwillingness to 

 admit that one species has given birth to other and 

 distinct species, is that we are always slow in admitting 

 any great change of which we do not see the interme- 

 diate steps. The difficulty is the same as that felt by 

 so many geologists, when Lyell first insisted that long 

 lines of inland cliffs had been formed, and great valleys 

 excavated, by the slow action of the coast-waves. The 

 mind cannot possibly grasp the full meaning of the 

 term of a hundred million years ; it cannot add up 

 and perceive the full effects of many slight variations, 

 accumulated during an almost infinite number of ge- 

 nerations. 



Although I am fully convinced of the truth of the 

 views given in this volume under the form of an 

 abstract, I by no means expect to convince experienced 

 naturalists whose minds are stocked with a multitude 

 of facts all viewed, during a long course of years', from 

 a point of view directly opposite to mine. It is so easy 



Y 



