56 THE EVOLUTION THEORY 



a position to understand, in some degree, the marvellous adaptation 

 of the organism to an end, without having to call to our aid any super- 

 naturally intrusive force on the part of the Creator. We understand 

 now how, in a purely mechanical way, through the forces always 

 at work in nature, all forms of life must conform to, and adapt 

 themselves precisely to the conditions of their life, since only the 

 best possible is preserved, and everything less good is continually 

 being rejected. 



Before I go on to expound in detail the phenomena which we refer 

 to natural selection, I must briefly state that Darwin did not ascribe 

 to natural selection by any means all the changes which have taken 

 place in organisms in the course of time. On the one hand, he 

 ascribed a not inconsiderable importance to the correlated variations 

 we have already mentioned ; still more, however, he relied on the 

 direct influence of altered conditions of life, whether these consist 

 in climatic and other changes in the environment, or in the assump- 

 tion of new habits, and the increased or diminished use of individual 

 parts and organs thereby induced. He recognized the principle 

 so strongly emphasized by Lamarck, of use and disuse as a cause 

 of heritable increase or decrease of the exercised or neglected part, 

 though he did so with a certain reserve. I shall return later to these 

 factors of modification, and shall then attempt to show that these 

 too are to be referred to processes of selection, which are, however, of 

 a different order from the phenomena which the Darwin-Wallace 

 principle of natural selection serves to interpret. But, in the first 

 instance, it appears to me to be necessary to show how far the 

 Darwin -Wallace interpretation will suffice, and in the next lectures 

 we shall occupy ourselves with this question exclusively. 



