1897] FUNDAMENTAL PRINCIPLES OF HEREDITY 313 



of an organ these biophors are distributed between the cells ; and 

 the proper biophors in each cell constrain it to play its specific part 

 in the organism. 



Those cells which constitute the direct line of descent between the 

 reproductive cells of one generation and those of another are formed 

 by true divisions of the germ-plasm, with all its determinants. But 

 we are met by the facts of propagation by fragments composed only 

 of tissue-cells in Animal, and still more in Plants, where specialised 

 tissue-cells revert to an embryonic condition, or rather beget 

 embryonic cells with a complete germ-plasm. To explain this diffi- 

 culty, we must suppose that in these cases a portion of complete 

 germ-plasm has passed at their formation into such tissue-cells, and 

 that it has remained dormant until the stimulus of separation from 

 the colonial organism has revived its vitality. Again in the four- 

 celled stage of the segmented embryo of various widely distinct 

 Animals (even in the sixteen-celled stage of some Medusae) it is 

 possible to isolate a single cell, which then develops into a complete 

 embryo, though had it remained associated with its fellows it would 

 have formed only a definite part of the embryo. Here again we find 

 the assumption of the existence of dormant determinants, which be- 

 come active only in the separated cell, adduced by Weismann to save 

 the theory. This assumption is also used to explain alternation of 

 generations, where the Moss-plant and Moss-urn, or the Fern-scale 

 and Fern-plant alternate ; their germ-plasm must contain two sets of 

 determinants, one for the first, the other for the second- generation, 

 alternating in sleep and waking like the printer and the hatter in 

 Box and Cox. We are reminded of the complex epicycles required 

 to render the universe workable on Ptolemy's geocentric hypothesis, 

 and the Spanish king's comment thereon. " Had I been consulted 

 at the creation, I could have simplified matters." 



So far indeed, this might be held as a formal or fictive hypo- 

 thesis to explain the mechanism of heredity on the basis of Special 

 Creation — each organism being created fully equipped with its 

 own proper germ-plasm, determinants, biophors, and all. But no ; 

 Weismann is a firm believer in the theory of common descent, and, 

 as we have seen, he and his school profess to be the only true 

 Darwinians ; and we come to his Theory of Variations. 1 



The germ-plasm with its contained determinants, as it lies 

 in the reproductive cells of the body, is subject to nutritive 

 changes, and consequently to constant slight variations which 

 apparently are not correlated with anything else whatever. The 

 haphazard variations, of the determinants induce corresponding, and 



1 This essay was written nearly two years ago. Since then Weismann has enlarged 

 his theory by the hypothesis of germinal selection. Without going into this we may 

 note that it makes no difference to the present argument. 



