CONCLUSION 305 



that Bpecies 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 thej 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 

 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 cliffs 

 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," etc., and to think that we 

 give an explanation when we only restate a fact. Any 

 one whose disposition leads him to attach more weight to 

 unexplained difficulties than to the explanation of a cer- 

 tain number of facts will certainly reject the theory. A 

 few naturalists, endowed with much flexibility of mind. 



