Chap. XXI. AND CONCLUDING EEMAKKS. 387 



consequently he is occasionally subjected to a severe 

 struo-Dfle for existence, and natural selection will have 

 effected whatever lies within its scope. A succession 

 of strongly-marked variations of a similar nature are 

 by no means requisite; slight fluctuating differences 

 in the individual suffice for the work of natural selec- 

 tion. We may feel assured that the inherited effects 

 of the long-continued use or disuse of parts will have 

 done much in the same direction with natural selection. 

 Modifications formerly of importance, though no longer 

 of any special use, will be long inherited. When one 

 part is modified, other parts will change through the 

 principle of correlation, of which we have instances in 

 many curious cases of correlated monstrosities. Some- 

 thing may be attributed to the direct and definite 

 action of the surrounding conditions of life, such as 

 abundant food, heat, or moisture ; and lastly, many 

 characters of slight physiological importance, some in- 

 deed of considerable importance, have been gained 

 through sexual selection. 



No doubt man, as well as every other animal, pre- 

 sents structures, which as far as we can judge with 

 our little knowledge, are not now of any service to 

 him, nor have been so during any former period of his 

 existence, either in relation to his general conditions of 

 life, or of one sex to the other. Such structures cannot 

 be accounted for by any form of selection, or by the 

 inherited effects of the use and disuse of parts. We 

 Ivuow, however, that many strange and strongly-marked 

 peculiarities of structure occasionally appear in our 

 domesticated productions, and if the unknown causes 

 which produce them were to act more uniformly, they 

 would probably become common to all the individuals 

 of the species. We may hope hereafter to understand 

 something about the causes of such occasional modi- 



2 c 2 



