Chap. XXI. General Summary. 607 



are facts which cannot be disputed. They have long been 

 known, but until recently they told us nothing with respect to 

 the origin of man. Now when viewed by the light of our know- 

 ledge of the whole organic world, their meaning is unmistakable. 

 The great prin>jiple of evolution stands up clear and firm, wheL 

 these groups of facts are considered in connection with others 

 such as the mutual affinities of the members of the same group, 

 their geographical distribution in past and prcseot times, and 

 their geological succession. It is incredible that all these facta 

 should speak falsely. 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 instance, of a dog — the construction 

 of his skull, limbs and whole frame on the same plan with that of 

 other mammals, independently of the uses to which the parts 

 may bn 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-descendant 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 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 naturnl 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 

 differences in the individual suffice for the work of natural selec- 

 tion ; not that we have any reason to suppose that in the same 

 species;, all parts of the organisation tend to vary to the same 

 degree. 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, are long- 

 inherited. When one part is modified, otht-r parts change 

 through the principle 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 

 moistTire; and lastly, many characters of slight physiological 



