72 THE DESCENT OF MAN 



in this case, and in all others, many checks concur, and 

 different checks under different circumstances; periodical 

 dearths, depending on unfavorable seasons, being probably 

 the most important of all. So it will have been with the 

 early progenitors of man. 



Natural Selection. — We have now seen that man is vari- 

 able in body and mind; and that the variations are induced, 

 either directly or indirectly, by the same general causes, and 

 obey the same general laws, as with the lower animals. Man 

 has spread widely over the face of the earth, and must have 

 been exposed, during his incessant migrations," to the most 

 diversified conditions. The inhabitants of Tierra del Fuego, 

 the Cape of Good Hope, and Tasmania in the one hemi- 

 sphere, and of the Arctic regions in the other, must have 

 passed through many climates, and changed their habits 

 many times, before they reached their present homes." 

 The early progenitors of man must also have tended, like 

 all other animals, to have increased beyond their means of 

 subsistence; they must, therefore, occasionally have been 

 exposed to a struggle for existence, and consequently to 

 the rigid law of natural selection. Beneficial variations of 

 all kinds will thus, either occasionally or habitually, have 

 been preserved, and injurious ones eliminated. I do not 

 refer to strongly marked deviations of structure, which 

 occur only at long intervals of time, but to mere individ- 

 ual differences. "We know, for instance, that the muscles 

 of our hands and feet, which determine our powers of move- 

 ment, are liable, like those of the lower animals," to inces- 

 sant variability. If then the progenitors of man inhabiting 

 any district, especially one undergoing some change in its 

 conditions, were divided into two equal bodies, the one-half 



" See some good remarks to this effect by W. Stanley Jevons, "A Deduc- 

 tion from Darwin's Theory," "Nature," 1869, p. 231. 



*^ Latham, "Man and his Migrations," 1851, p. 135, 



*' Messrs. Murie and Mivart in their ' 'Anatomy of the Lemuroidea' ' (' 'Trans- 

 act. Zoolog. Soo.,"to1. vii. 1869, pp. 96-98) say: "Some muscles are so irregular 

 in their distribution that they cannot be well classed in any of the above groups. " 

 These muscles diifer even on the opposite sides of the same individual. 



