xx THE INFLUENCE OF ENVIRONMENT 403 



success of the development depends on the availability 

 of an appropriate nurture, the details of which are often 

 very precise. In his work on The Mechanism of Mende- 

 lian Heredity (1915), Prof. T. H. Morgan emphasises the 

 mutual dependence of nature and nurture : a " character is 

 the product of a number of genetic factors and of environ- 

 mental conditions " ; " every character is the realised 

 result of the reaction of hereditary factors with each 

 other and with their environment " ; or again, ' it is a 

 commonplace that the environment is essential for the 

 development of any trait, and that traits may differ 

 according to the environment in which they develop." 



While the strength of an (inherited) individuality may 

 be such that it expresses itself almost in the face of 

 inappropriate nurture, there is a minimum nurture neces- 

 sary if there is to be any development at all ; and the 

 conditions of nurture determine whether the expression 

 of the inheritance is to be full or partial, abundant or 

 stunted, or, it may be, as regards a particular feature, 

 absent altogether. 



Gudernatsch has shown that in tadpoles fed on thyroid 

 there is differentiation without growth, while in tadpoles 

 fed on thymus and spleen there is growth without differen- 

 tiation. A character known to be part of the inheritance 

 may remain entirely unexpressed in the individual 

 development because certain environmental conditions 

 are lacking, yet the heritable character may be handed on 

 all the same. Thus fruit-flies (Drosophila) of a Mendelian 

 race with a peculiar abnormality may appear perfectly 

 normal if raised in a dry bottle, but the presence within 

 them of the " factor ' for abnormal may be demon- 

 strated by rearing their offspring in a wet bottle. 



A diagrammatic illustration concerns the red Chinese 

 primrose (Primula sinensis rubra). Reared at 15-20 C. 

 it has red flowers ; reared at 30-35 C., with moisture 

 and shade, the same plants have pure white flowers like 

 those of Primula sinensis alba, which always has white 

 flowers. The development of colour in the red Chinese 

 primrose depends on its nurture. 



Take another illustration from the fruit-fly. There 



