40 THE EVOLUTION THEOKY 



into each daughter-nucleus. It is the true reducing division, through 

 which the number of the rods is reduced to one half \ 



This numerical reduction must, however, have other consequences ; 

 it must make the germ-cells of the same individual qualitatively 

 unlike, that is, in relation to their value in inheritance. Let us assume 

 only four chromosomes of the rod-form (' idants ') as the nuclear 

 elements of a species, two of which, A and 5, come from the mother, and 

 other two, G and D, from the father, the la.st maturing division 

 may, as far as we can see, result either in renio\'ing the combination 

 A and B from C and D, or A and C from B and D,oy A and D from B and 

 G ; there is thus a possibility of one of six different combinations of rods 

 in any one germ-cell. What is the same thing, six different kinds of 

 germ-cells differing in their hereditary primary constituents may be 

 developed in the same individual. As this new combination, or, as we 

 may call it, neotaxis of the germ-plasm elements, takes place in female 

 as well as in male individuals, there is a possildlity that, in fertiliza- 

 tion, 6x6 = ^6 individuals with different primary constituents may 

 arise from the germ-cells of the same two parents. Of course the 

 number of possible combinations increases very considerably in pro- 

 portion to the normal number of rods, for with eight of these it conies 

 up to 70, and with sixteen to 12,870 ; the number of individuals differing 

 in their inherited primary constituents would tluis be enormous, for 

 each of the 70 or of the 12,870 different hereditary minglings of the 

 ovum could combine in amphimixis with 70 or 12,870 different 

 sperm-cells, so that 70 x 70 and 12,870 x 12,870 offspring individually 

 different in their primary constituents might arise from the same two 

 parents. In Man there are said to be sixteen nuclear rods ; so that in 

 his case the last-mentioned numbei* of parental hereditary minglings 

 might occur. This may seem a disproportionately high number as 

 compared with the small number of children of a human pair, but we 

 must not judge from the case of Man alone, and in plants and animals, 

 which we have already discussed, the number of descendants is very 

 much larger, and is often enormous. We saw what significance this 

 apparent extravagance on the part of nature has, for without it adapta- 

 tion to changed conditions of life would not be possible, since, if only 

 so many were born as could attain to reproduction, no selection of the 

 fittest could take place. The same would be the case if all the young 

 of a species were alike, and even if all the descendants of a single pair 



^ Eecent investigations have shown that the reduction of the chromosomes does 

 not always take phice exactly in accordance with the scheme here indicated, but that 

 it differs from it in many cases. But as investigations on this point are as yet by no 

 means complete, I need not go into the question further; the ultimate result is the 

 same in any case. 



