602 THE DESCENT OF MAN. 



He who is not content to look, like a savage, at the phenomena 

 of nature as disconnected, cannot any longer believe that man is 

 the work of a separate act of creation. He will be forced to admit 

 that the close resemblance of the embryo of man to that, for in- 

 stance of a dog — the construction of his skull, limbs and whole 

 frame on the same plan with that of other mammals, independent- 

 ly of the uses to which the parts may be put — the occasional re- 

 appearance of various structures, for instance of several muscles, 

 which man does not normally possess, but which are common to 

 the Quadrumana — and a crowd of analogous facts — all point in 

 the plainest manner to the conclusion that man is the co-descend- 

 ant with other mammals of a common progenitor. 



We have seen that man incessantly presents individual differ- 

 ences In all parts of his body and in his mental faculties. These 

 differences or variations seem to be induced by the same general 

 causes, and to obey the same laws as with the lower animals. 

 In both cases similar laws of inheritance prevail. Man tends to 

 increase at a greater rate than his means of subsistence; con- 

 sequently he is occasionally subjected to a severe struggle for 

 existence, and natural selection will have effected whatever lies 

 within its scope. A succession of strongly-marked variations of a 

 similar nature is by no means requisite; slight fluctuating differ- 

 ences in the individual suffice for the work of natural selection; 

 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 effects of the long-con- 

 tinued use or disuse of parts will have done much in the same 

 direction with natural selection. Modifications formerly of im- 

 portance, though no longer of any special use, are long-inherited. 

 When one part is modified, other parts change through the prin- 

 ciple of correlation, 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 conditions of 

 life, such as abundant 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 struct- 

 ures, which seem to our limited knowledge, not to be now of any 

 service to him, nor to have been so formerly, either for the gen- 

 eral 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 peculiar- 

 ities of structure occasionally appear in our domesticated produc- 

 tions, 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 



