782 THE DESCENT OF MAN 



not that we have any reason to suppose that, in the same 

 species, all parts of the organization tend to vary to the 

 same degree. We may feel assured that the inherited ef- 

 fects 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, are long inherited. When one part is 

 modified, other parts change through the principle of cor- 

 relation, of which we have instances in many curious cases 

 of correlated monstrosities. Something may be attributed 

 to the direct and definite action of the surrounding condi- 

 tions of life, such as abundant food, heat, or moisture; and 

 lastly, many characters of slight physiological importance, 

 some indeed of considerable importance, have been gained 

 through sexual selection. 



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

 structures which seem to our limited knowledge not to be 

 now of any service to him, nor to have been so formerly, 

 either for the general conditions of life or in the relations 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 occa- 

 sionally appear in our domesticated prodactionSj and if their 

 unknown causes 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 modifications, especially 

 through the study of monstrosities ; hence the labors of ex- 

 perimentalists, such as those of M. Camille Dareste, are full 

 of promise for the future. In general we can only say that 

 the cause of each slight variation and of each monstrosity 

 lies much more in the constitution of the organism than in 

 the nature of the surrounding conditions; though new and 

 changed conditions certainly play an important part in ex- 

 citing organic changes of many kinds. 



Through the means just specified, aided perhaps by oth- 



