CHAPTER VIII. 



CHARACTERS AS ADAPTIVE AND SPECIFIC 

 (continued). 



LET us now proceed to indicate some of the 

 causes, other than natural selection, which may be 

 regarded as adequate to induce such changes in 

 organic types as are taken by systematists to con- 

 stitute diagnostic distinctions between species and 

 species. We will first consider causes external to 

 organisms, and will then go on to consider those which 

 occur within the organisms themselves : following, in 

 fact, the classification which Darwin has himself laid 

 down. For he constantly speaks of such causes as 

 arising on the one hand, from ' changed conditions of 

 life " and, on the other hand, from " the nature of the 

 organism " that is, from internal processes leading 

 to " variations which seem to us in our ignorance to 

 arise spontaneously." 



In neither case will it be practicable to give more 

 than a brief rtsumt of all that might be said on these 

 interesting topics. 



I. Climate. 



There is an overwhelming mass of evidence to 

 prove that the assemblage of external conditions of 

 life conveniently summarized in the word Climate, 



