Heredity and Natural Selection. 287 



rect action of external conditions, and especially the law 

 of correlated variability. 



Our theory of heredity furnishes exactly what we need 

 to escape this difficulty, for we can understand !hat a 

 change in any part of the body, disturbing, as it must, 

 the harmonious adjustment of related parts, acts direct- 

 ly to produce variations in these parts in succeeding 

 generations, by causing the transmission of gemmules. 

 The time which is needed for the evolution of a compli- 

 cated organ by natural selection is thus brought within 

 reasonable limits, and one of the most serious and fun- 

 damental objections to Darwin's explanation of the ori- 

 gin of species is completely done away with. 



He sa3's: **We may borrow an illustration irom Mr. 

 Herbert Spencer, who remarks that when the Irish elk 

 acquired its gigantic horns, weighing above one hundred 

 pounds, numerous co-ordinated changes of structure 

 would have been indispensable, namely, a thickened skull 

 to carry the horns; strengthened cervical vertebrae with 

 strengthened ligaments; enlarged dorsal vertebrse to sup- 

 port the neck, with powerful fore-legs and feet; all these 

 parts being supplied with proper blood-vessels, muscles 

 and nerves. How, then, could these admirably co-ordin- 

 ated structures have been acquired? According to the 

 doctrine which I maintain, the horns of the male elk were 

 slowly gained through sexual selection, that is, by the 

 best armed males conquering the worse armed, and leav- 

 ing a greater number of descendants. But it is not at all 

 necessary that the several parts of the body should have 

 simultaneously varied. Each stag presents individual 

 differences, and in the same district those which had 

 slightly heavier horns, or stronger necks, or stronger 

 bodies, or were the most courageous, would serve the 

 greatest number of does, and consequently have the 



