INTRODUCTORY 1 .3 



but we shall devote much space to a discussion 

 of the causes of variation, recognising that this 

 question is absolutely fundamental to the theory of 

 oroanic evolution. 



And here it is proper to insist upon the im- 

 portance of our subject. Heredity is, of course, 

 a very interesting matter, and so is the occur- 

 rence of variation side by side with heredity. 

 But we study these things to-day in a very different 

 attitude from that of, say, a hundred years ago; 

 for we know their signiticance, their bearing upon 

 much greater matters. The astronomer of the 

 fifteenth or sixteenth century might be interested 

 in tracing the exact movements of a planet. 

 These were facts of Nature ; but, if they had any 

 ulterior significance, it was only in illustrating the 

 habits and customs of the great Spirits which even 

 Kepler supposed to inhabit and drive the heavenly 

 bodies. But nowadays, thanks to Kepler and 

 Galileo and Newton, the astronomer knows the 

 laws of motion and gravitation, and the movements 

 of a planet mean something for him. Similarly 

 the facts of heredity and variation mean something 

 for the biologist of to-day, and he studies them 

 hardly for themselves at all, but because of their 

 bearing on organic evolution. Now organic evo- 

 lution is not only the leading fact of biology, but 

 is the basis on which Herbert Spencer re-erected 

 the sciences that spring from biology, such as the 

 science of mind, the science of society, and the 

 science of morality. Hence the importance of 

 heredity and variation is cardinal, their elucida- 

 tion constituting a very corner-stone in that great 



