ON HEREDITY. 103 



and stunted ; life on board ship, with plenty of exercise and sea 

 air, gives the sailor bodily strength and a tanned skin ; but when 

 once the resemblance to father or mother, or to both, is established 

 in the germ-cell it can never be effaced, let the habit of life be 

 what it will. 



But if the essential nature of the germ-cell dominates over the 

 organism which will grow from it, so also the quantitative in- 

 dividual differences, to which I referred just now, are, by the same 

 principle, established in the germ, and whatever be the cause 

 which determines their presence they must be looked upon as 

 inherent in it. It therefore follows that, although natural selection 

 appears to operate upon the qualities of the developed organism 

 alone, it in truth works upon peculiarities which lie hidden in the 

 germ-cells. Just as the final development of any predisposition 

 in the germ, and just as any character in the mature organism 

 vibrates with a certain amplitude around a fixed central point, 

 so the predisposition of the germ itself fluctuates, and it is on 

 this that the possibility of an increase of the predisposition in 

 question, and its average result, depends. 



If we trace all the permanent hereditary variations from 

 generation to generation back to the quantitative variations 

 of the germ, as I have sought to do, the question naturally 

 occurs as to the source from which these variations arose in 

 the germ itself. I will not enter into this subject at any length 

 on the present occasion, for I have already expressed my opinion 

 upon it 1 . 



I believe however that they can be referred to the various ex- 

 ternal influences to which the germ is exposed before the com- 

 mencement of embryonic development. Hence we may fairly 

 attribute to the adult organism influences which determine the 

 phyletic development of its descendants. For the germ-cells are 

 contained in the organism, and the external influences which affect 

 them are intimately connected with the state of the organism in 

 which they lie hid. If it be well nourished, the germ-cells will 

 have abundant nutriment ; and, conversely, if it be weak and 

 sickly, the germ-cells will be arrested in their growth. It is even 



1 Consult ' Studien zur Descendenztheorie, IV. liber die mechamsche Auffassung 

 der Natur,' p. 303, etc. Translated and edited by Professor Meldola ; see ' Studies 

 in the Theory of Descent,' p. 677, &c. 



