610 THE CONTROL OF LIFE: 



differentiation. A character known to l>e part of the in- 

 heritance 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 nor- 

 mal if raised in a dry environment, but the presence within 

 them of the ' factor ' for the abnormal feature may be 

 demonstrated by rearing their offspring in a damp place. 

 This shows the importance of nurture for the individual. 



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. 

 Thus we see that the development of colour in the red Chinese 

 primrose depends on its nurture. 



Take another illustration from the fruit-fly. There is a 

 mutant stock that produces supernumerary legs, in consider- 

 able percentage in winter, few or none in summer. Miss 

 Hoge finds that when the flies are kept in an ice-chest at 

 a temperature of about 10 C., a high percentage of individ- 

 uals with supernumerary legs occurs. In a hot climate there 

 would be no evidence that the peculiarity was part of the 

 inheritance; in a cold country it would be obvious. This 

 shows that the expression of the inheritance as regards a 

 particular character sometimes depends on nurture. 



In estimating the importance of nurture for the individ- 

 ual man, we must remember how largely the human mind 

 is a social product. As Prof. George H. Parker (1914) 

 puts it, " Our intellectual outfit comes to us more in the 

 nature of a social contribution than an organic one." Per- 



