WEISMANN'S THEORY OF HEREDITY 173 



it is quite possible that there may be a number of ids of the same 

 kind. When, during the process of maturation, half the total 

 number of chromosomes (each made up of one or more ids) are 

 eliminated from the germ cell, it appears to be largely a matter 

 of chance which shall go and which shall remain, and the nature 

 of the new combination of ids resulting from the process of 

 amphimixis must also be a matter of chance, depending upon 

 what luck the germ cells happen to have in their mating. 



As has been well said, a new shuffling of the cards must take 

 place in each generation. The characters of the organism 

 developed from any zygote will depend upon the hand dealt out 

 to it in the processes of reduction and amphimixis, and as it can 

 rarely, if ever, happen that any two hands will be exactly alike, so 

 it will rarely, if ever, happen that any two organisms, however 

 closely related, will exactly resemble one another in all their 

 characters. Indeed, the only cases in which even an approxima- 

 tion to exact resemblance is known, at any rate amongst the 

 higher animals, are those of "identical" twins, and the explana- 

 tion of these is that they have arisen by an integral division of a 

 single fertilized ovum, followed by separation of the first two 

 daughter cells or blastomeres thus produced. 



We therefore find in the processes of reduction and amphimixis, 

 in the permutation and combination of ancestral characters, an 

 abundant source of variation. This, however, is not supposed 

 to explain fully the origin of variations, and Weismann accord- 

 ingly invokes the aid of another hypothesis, his theory of 

 " Germinal Selection." In accordance with this theory the 

 determinants of which the ids are composed are supposed to be 

 differently situated with regard to their facilities for obtaining the 

 nutriment necessary for their growth and multiplication. There 

 is a kind of struggle for existence going on amongst them. Those 

 which are more successful in obtaining supplies, having once got 

 a start, will tend to supplant those which are less successful. 

 Some will become weaker and some stronger, and thus, as the 

 result of differences in nutrition, variation is set up amongst the 

 determinants themselves. If these vary it follows that their 

 determinates, or the parts which they control in the developing 

 organism, will vary also. 1 



1 There would seem, however, to be a serious objection to the theory of germinal 

 selection in the fact that the nucleus of any given germ cell contains many ids, and 

 that similar determinants must as a rule recur in each id. We can hardly suppose 

 that the corresponding determinants in each id are always subject to precisely the 



