AUTHOES PKEFACE ix 



follow, and thus controls all differentiation, all ascent of 

 organization, and ultimately the whole course of organic 

 evolution on the earth, for everjrthing about Hving beings 

 depends upon adaptation, though not on adaptation in the 

 sense in which Darwin used the word. 



The great prominence thus given to the idea of selection 

 has been condemned as one-sided and exaggerated, but the 

 physicist is quite as open to the same reproach when he 

 thinks of gravity as operative not on our earth alone, but 

 as dominating the whole cosmos, whether visible to us or 

 not. If there is gravity at all it must prevail everywhere, 

 that is, wherever material masses exist ; and in the same way 

 the co-operation of certain conditions with certain primary 

 vital forces must call forth the same process of selection 

 wherever living beings exist ; thus not only are the vital 

 units which we can perceive, such as individuals and cells, 

 subject to selection, but those units the existence of which 

 we can only deduce theoretically, because they are too minute 

 for our microscopes, are subject to it likewise. 



This extension of the principle of selection to all grades 

 of vital units is the characteristic feature of my theories ; 

 it is to this idea that these lectures lead, and it is this— 

 in my own opinion — which gives this book its importance. 

 This idea will endure even if everything else in the book 

 should prove transient. 



Many may wonder, perhaps, why in the earlier lectures 

 much that has long been known should be presented afresh, 

 but I regard it as indispensable that the student who wishes 

 to make up his own mind in regard to the selection-idea 

 should not only be clear as to what it means theoretically, 

 but should also form for himself a conception of its sphere 

 of influence. Many prejudiced utterances in regard to 

 'Natural Selection' would never have been published if 

 those responsible for them had known more of the facts ; 

 if they had had any idea of the inexhaustible wealth of 

 phenomena which can only be interpreted in the light of this 

 principle, in as far, that is, as we are able to give explanations 



