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Show 386 GENETIAJ, SUMMARY PART II~ he retains, and the abnormal reversions to which he is occasionally liable,-are facts which cannot be disputed. 'rhey 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 knowledge of the whole organic world, their meaning is unmistakeable. The great principle of evolution stands up clear and firm, when 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 present times, and their geological succession. It is incredible that all these facts 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 wm 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, indepen-· dently of the uses to which the parts may be put, on the same plan with that of other mammals-the occasional reappearance of various structures, for instance of several distinct 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 differences in all pn.rts of his body and in his. mental faculties. 'rhese 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;. CHAP. XXI. AND CONCLUDING TIEMARKS. 387 consequently 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 are by no means requisite ; slight fluctuating differences in the individual suffice 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 inherited. When one part is modified, other parts will change through the principle of correlation, of which wo 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 considerable importance, have been gained through sexual selection. No doubt man, as 'vell 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 dm·ing 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 usc 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 occasional modi- 2 c 2 |