SHARE OF THE PARENTS IN BUILDING UP THE OFFSPRING 41 



were alike, effective selection would be excluded, since only as many 

 individualities could be selected as there were pairs of parents. It is 

 easy to understand that selection works more effectively the larger 

 the number of descendants of a species and the more they differ from 

 each other. The chance that the best possible combination of characters 

 will occur is thereby increased. 



Although we cannot calculate how many individuals of different 

 combinations of characters natural selection requires to work upon in 

 order to direct the evolution of the species ^, we can understand that 

 only as large a choice as possible can secure that the best possible 

 adaptations of all parts and organs are brought about and main- 

 tained. Precisely in the fact that in every generation such an 

 enormous superfluity of individuals is produced lies the possibility 

 of such intensive processes of selection as must continually take place, 

 if the adaptation of all parts is to be explained. For if among the 

 thousands of descendants of a fertile species each hundred were alike 

 among themselves, these hundreds would have, as far as natural 

 selection was concerned, only the value of a single variant. But such 

 an all-round adaptation as actually exists in the structure of species 

 requires as many variants as possible ; it requires that each individual 

 should be a 'pecidiar coiwplex of hereditary characters ; that is, that 

 all the fertilized germ-cells of a pair should possess an individually 

 well-marked character. 



The justification of this postulate becomes all the clearer if we 

 take into consideration the male germ-cells as well as the female. 

 Let us think of the enormous number of sperm-cells which are 

 produced by many animals, and indeed by the highest of them — an 

 almost incalculably large number which certainly goes far beyond 

 millions. Let us assume that in Man there may be 12,870 million 

 spermatozoa, then, with sixteen ids, and with an equally frequent 

 occurrence of all possible combinations of germ-plasm — there would 

 be 12,870 — there would be a million of each type containing 

 identical germ-plasm. The danger that several ova would be 

 fertilized by identical sperm-cells would be by no means small. 



It cannot, therefore, surprise us that other means have been 

 employed by Nature to secure re-groupings of the ids. The simplest 

 means would be, if before each division of the primitive germ-cells 

 the nuclear rods were to divide, and if the split halves were 



1 For this reason I have left the number of id-combinations given above unaltered, 

 tliougli, according to tlie most recent researches into the processes of maturation, they 

 are probably too high, since every conceivable combination does not actually 

 occur. We are here concerned less with the exact number than with the 

 principle. 



