370 GENERAL SUMMARY [Part H. 



and natural selection will have effected whatever lies with- 

 in its scope. A succession of strongly-marked variations 

 of a similar nature arc by no means requisite; slight 

 fluctuating ditfcrences in the individual suflice for the 

 work of natural selection. 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 inher- 

 ited. When one part is modified, other parts will change 

 through the principle of correlation, of which we have in- 

 stances in many cm-ious cases of correlated monstrosities. 

 Something may be attributed to the direct and definite 

 action of the surrounding conditions of life, such as abun- 

 dant food, heat, or moisture ; and lastly, many characters 

 of slight physiological importance, some indeed of con- 

 siderable importance, have been gained through sexual 

 selection. 



No doubt man, as well as every other animal, presents 

 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 know, 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 occa- 

 sional modifications, especially through the study of mon- 

 strosities : hence the labors of experimentalists, such as 

 those of M. Camille Dareste, are full of promise for the 



