THE PERIOD OF INCUBATION 67 



that remarkable book formed a crisis and turning-point 

 in liis mental development we know from his own 

 distinct statement in a letter to Haeckel, prefixed to 

 the brilliant German evolutionist's ' History of Creation.' 

 ' It seemed to me probable,' says Darwin, speaking of 

 his own early development, ' that allied species were 

 descended from a common ancestor. But during several 

 years I could not conceive how each form could have 

 been modified so as to become admirably adapted to its 

 place in nature. I began therefore to study domesti- 

 cated animals and cultivated plants, and after a time 

 perceived that man's power of selecting and breeding 

 from certain individuals was the most powerful of all 

 means in the production of new races. Having attended 

 to the habits of animals and their relations to the sur- 

 rounding conditions, I was able to realise the severe 

 struggle for existence to which all organisms are sub- 

 jected ; and my geological observations had allowed me 

 to appreciate to a certain extent the duration of past 

 geological periods. With my mind thus prepared I 

 fortunately happened to read Malthus's " Essay on Popu- 

 lation ; " and the idea of natural selection through the 

 struggle for existence at once occurred to me. Of all the 

 subordinate points in the theory, the last which I under- 

 stood was the cause of the tendency in the descendants 

 from a common progenitor to diverge in character.' 



It is impossible, indeed, to overrate the importance 

 of Malthus, viewed as a schoolmaster to bring men to 

 Darwin, and to bring Darwin himself to the truth. 

 Without the ' Essay on the Principle of Population ' it is 

 quite conceivable that we should never have had the 

 4 Origin of Species ' or the ' Descent of Man.' 



