v.] IN THE THEORY OF NATURAL SELECTION. 283 



the very beginning of development, and do not solely depend 

 upon the accidental conditions under which they live. More- 

 over, no one of the descendants can be identical with any of 

 the ancestors, for each of the former unites within itself the 

 hereditary tendencies of two parents, and its organism is there- 

 fore, as it were, a compromise between two developmental 

 tendencies. Similarly in the third generation, the hereditary 

 tendencies of two individuals of the second generation enter 

 into combination. But since the germ-plasm of the latter is not 

 simple, but composed of two individually distinct kinds of germ- 

 plasm, it follows that an individual of the third generation is a 

 compromise between four different hereditary tendencies. In 

 the fourth generation, eight ; in the fifth, sixteen ; in the sixth, 

 thirty-two difterent hereditary tendencies must come together, 

 and each of them will make itself more or less felt in some part 

 of the future organism. Thus by the sixth generation a large 

 number of varied combinations of ancestral individual characters 

 will appear, combinations which have never existed before and 

 which can never exist again. 



We do not know the number of generations over which the 

 specific hereditary tendencies of the first generation can make 

 themselves felt. Many facts seem to indicate however that the 

 number is large, and it is at all events greater than six. When 

 we remember that, in the tenth generation, a single germ con- 

 tains 1024 different germ-plasms, with their inherent hereditar}^ 

 tendencies, it is quite clear that continued sexual reproduction 

 can never lead to the re-appearance of exactly the same com- 

 bination, but that new ones must always arise. 



New combinations are all the more probable because the dif- 

 ferent idioplasms composing the germ-plasm in the germ-cells 

 of any individual are present in different degrees of intensity at 

 different times of its life ; in other worlds, the intensity of the 

 component idioplasms is a function of time. This conclusion 

 follows from the fact that children of the same parents are 

 never exactly identical. In one child the characters of the 

 father may predominate, in another those of the mother, in 

 another again those of either grand-parent or great-grand- 

 parent. 



We are thus led to the conclusion that even in a few sexually 

 produced generations a large number of well-marked individuals 



