104 ON HEREDITY. [II. 



moulded into almost any imaginable form in the course of 

 time. 



If we ask in what lies the cause of this variability, the answer 

 must undoubtedly be that it lies in the germ-cells. From the 

 moment when the phenomena which precede segmentation 

 commence in the ^g^, the exact kind of organism which will be 

 developed is already determined — whether it will be larger or 

 smaller, more like its father or its mother, which of its parts 

 will resemble the one and which the other, even to the minutest 

 detail. In spite of this, there still remains a certain scope for 

 the influence of external conditions upon the organism. But 

 this scope is limited, and forms but a small area round the 

 fixed central point which is determined by heredity. Abundant 

 nourishment can make the body large and strong, but can never 

 make a giant out of the germ-cell destined to become a dwarf 

 Unhealthy sedentary habits or insufficient nourishment makes 

 the factory-hand pale 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 the 

 organism which will grow from it, so also the quantitative 

 individual 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 quali- 

 ties 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 



