108 ZOOLOGICAL PHILOSOPHY 



environment of any race of animals induces new habits in these 

 animals. 



Now, if a new environment, which has become permanent for some 

 race of animals, induces new habits in these animals, that is to say, 

 leads them to new activities which become habitual, the result will 

 be the use of some one part in preference to some other part, and in 

 some cases the total disuse of some part no longer necessary. 



Nothing of all this can be considered as hypothesis or private opinion ; 

 on the contrary, they are truths which, in order to be made clear, only 

 require attention and the observation of facts. 



We shall shortly see by the citation of known facts in evidence, 

 in the first place, that new needs which estabhsh a necessity for some 

 part really bring about the existence of that part, asaresult.ofefforts ; 

 and that subsequently its continued use gradually strengthens, 

 develops and finally greatly enlarges it ; in the second place, we shall 

 see that in some cases, when the new environment and the new needs 

 have altogether destroyed the utihty of some part, the total disuse 

 of that part has resulted in its gradually ceasing to share in the 

 development of the other parts of the animal ; it shrinks and wastes 

 little by little, and ultimately, when there has been total disuse for 

 a long period, the part in question ends by disappearing. All this is 

 positive ; I propose to furnish the most convincing proofs of it. 



In plants, where there are no activities and consequently no habits, 

 properly so-called, great changes of environment none the less lead to 

 great differences in the development of their parts ; so that these 

 differences cause the origin and development of some, and the shrinkage 

 and disappearance of others. But all this is here brought about by 

 the changes sustained in the nutrition of the plant, in its absorption 

 and transpiration, in the quantity of calorig, light, air and moisture 

 that it habitually receives ; lastly, in the dominance that some of the 

 various vital movements acquire over others. 



Among individuals of the same species, some of which are continually 

 well fed and in an environment favourable to their development, 

 while others are in an opposite environment, there arises a difference 

 in the state of the individuals which gradually becomes very remark- 

 able. How many examples I might cite both in animals and plants 

 which bear out the truth of this principle ! Now if the environment 

 remains constant, so that the condition of the ill-fed, suffering or 

 sickly individuals becomes permanent, their internal organisation is 

 ultimately modified, and these acquired modifications are preserved 

 by reproduction among the individuals in question, and finally give 

 rise to a race quite distinct from that in which the individuals have been 

 continuously in an environment favourable to their development. 



