WEISMANN'S THEORY 137 



as made up of a vast number of ancestral plasms, or "ids," 

 derived from the long line of progenitors. 



I need not here explain his most ingenious demonstration 

 of the means whereby at each successive act of fertilization a 

 certain number of these ancestral ids are discharged, so that the 

 ovum and spermatozoon each contains half the number originally 

 present, and so that the number of ids in the fertilized ovum is 

 kept constant. I need but point out, in passing, that by this 

 process of reduction the set of ids discharged from one germ 

 cell is by this theory held to be different from that discharged 

 from another germ cell of the same individual. And as the 

 ids in two fertilized ova are not identical, as the same series of 

 ancestors do not contribute to the germ plasm of the two, so 

 it is that individual variation originates ; the ids varying, the 

 individuals controlled or developed by these ids tend to vary. 

 So it is by this fortuitous commingling that spontaneous varia- 

 tion is apt to show itself. 



Further, it has to be noted that when the ovum undergoes 

 segmentation, and half of the nuclear material passes into one 

 cell, half into the other, according to Weismann a certain definite 

 series of these hypothetical ids passes into each cell, and accord- 

 ing to the series entering each of them, so (and not by any varia- 

 tion in environmental influence) are determined the eventual 

 characters of the tissues to which those daughter cells eventually 

 give rise. The germ cells of the new individual become differ- 

 entiated at a very early period, receive the full complement of 

 the ids, and so carry on the whole series of properties inherited 

 from the ancestors. 



Difficulties in accepting this Theory. — This is a crude recapitula- 

 tion of the main points of Weismann's theory. I have but 

 mentioned those portions which especially bear upon my argu- 

 ment. To explain atavism, or the reproduction of characters 

 in one generation which had not been recognizable in the previous 

 generation but had been present in some earlier generations, 

 you will see that the theory demands that some at least of these 

 ancestral ids should have remained unchanged through a long 

 period — it may be for centuries. For Darwin's case 1 of the 

 return of features peculiar to the ancestral rock pigeon, brought 

 about by crossing barbs, spots, and fantails, is clearly a case of 



1 Darwin, Origin of Species, sixth edition, p. 18. 



