Chap. XV.] CONCLUSION: 295 



invariably sterile, and varieties invariably fertile; or 

 that sterility is- a special endowment and sign of creation, 

 the belief that species were immutable productions 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 mu- 

 tation. 



But the chief cause of our natural unwillingness to 

 admit that one species has given birth to clear and dis- 

 tinct species, is that we are always slow in admitting 

 great changes of which we do not see the steps. The 

 difficulty is the same as that felt by so many geologists, 

 when Lyell first insisted that long lines of inland cliifs 

 had been formed, and great valleys excavated, by the 

 agencies which we see still at work. The mind cannot 

 possibly grasp the full meaning of the term, of even a 

 million years; it cannot add up and perceive the full 

 effects of many slight variations, accumulated during an 

 almost infinite number of generations. 



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 to hide 

 our ignorance under such expressions as the " plan of 

 creation," " unity of design," &e., and to think that we 

 give an explanation when we only re-state a fact. Any 

 one whose disposition leads him to attach more weight 

 to unexplained difficulties than to the explanation of a 



