4 6a HEREDITY AND DEVELOPMENT 



Weismann, we must refer those individual germinal variations 

 which form part of the raw material of evolution. But it can 

 hardly be imagined that all the determinants or hereditary 

 constituents are equally vigorous, or have equal assimilating 

 power. Thus, a determinant may become weaker because 

 there is less food for it, and also because it has less power of 

 utilising the available food. If a determinant is thus weakened, 

 its determinate the structure to which it corresponds will 

 also be weakened ; and we call this a germinal variation on the 

 down-grade. On the other hand, a vigorous determinant with 

 strong assimilative power will tend to become stronger if it 

 is well and appropriately fed. Its determinate will be 

 correspondingly strengthened, and we call this a germinal 

 variation on the up-grade. 



" To the ascending progression there are limits set, not only 

 by the amount of food which can circulate through the whole 

 id (a complete system of determinants), but also by the neigh- 

 bour determinants, which will sooner or later resist the with- 

 drawal of nourishment from them ; but for the descending 

 progression there are no limits except total disappearance, 

 and this is actually reached in cases in which the determinants 

 are related to a part which has become useless " (Weismann, 

 1904, vol. ii. p. 118). 



" If the germ-plasm be a system of determinants, then 

 the same laws of struggle for existence in regard to food and 

 multiplication must hold sway among its parts that obtain 

 between all systems of vital units among the biophors which 

 form the protoplasm of the cell-body, among the cells of a tissue, 

 among the tissues of an organ, among the organs themselves, 

 as well as among the individuals of a species and between species 

 which compete with one another." 



When a structure becomes useless in the life of a species, 

 those individuals who have more of it are no better off than 

 those who have less of it ; natural selection no longer operates 



