CONCLUSION. 497 



tlie tniitability of species? It cannot be asserted that 

 organic beings in a state of nature are subject to no varia- 

 tion; it cannot be proved that the amount of variation in 

 the course of long ages is a limited quantity; no clear dis- 

 tinction 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 immut- 

 able 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 recoi'd 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 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 still see 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, dur- 

 ing a long course of years, from a point of view directly op- 

 posite to mine. It is so easy to hide our ignorance under 

 such expressions as the "plan of creation," "unity of de- 

 sign," 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 certain number of facts will certainly 

 reiect the theory. A few naturalists, endowed with much 

 flexibility of mind, and who have already begun to doubt 

 the immutability of species, may be influenced by this vol- 

 ume; but I look with confidence to the future, to young 



