THE METHOD 81 



natural condition is designed for the purpose of pro- 

 tecting the shorn lamb, or any other organism. When 

 a considerable change occurs in any region inhabited 

 by animals, some of them may be so organized as to 

 survive the change. Some of them may migrate to 

 other regions, adapted to their existing forms. But 

 whatever occurs in the complex and sometimes slow 

 changes of both animal life and environment, the life 

 forms must, in order to survive, become adapted to the 

 environment, unless they happen to be already so 

 adapted. 



The mutual correspondence between the animal and 

 its environment brings about, what we call an evolution 

 in the order of all life forms, by which, the necessary 

 forms of present races correspond with the present 

 natural conditions of the earth's surface; and the 

 great differentiation in species, now inhabiting the 

 earth, is accounted for, in a logical manner, by descent 

 with modification. Not only the observations of 

 naturalists on the methods of nature, but the artificial 

 experiments of breeders, prove that species are mutu- 

 able, at least within certain limits of variability. 



UNIVERSALITY OF NATURAL SELECTION. Persistent 

 types, being so well adapted to every change in the en- 

 vironment, have no variation because none of the sup- 

 posed causes of variation, inherent tendency, polarity, 

 mutation, sexual selection, use or non-use, and the more 

 potent of all, external conditions, which means simply 

 environment, is powerless to effect any change of form. 

 But natural selection operates here as elsewhere, in keep- 

 ing a form so well adapted in the adapted form, with- 

 out the necessity of variation. In other words, natural 

 selection is the principle of adaptation, and is equally 

 efficient in sudden, or slow, and minute variations, or in 



