EVOLUTION 985 



which to refer; yet we think that what we have to say may be 

 subscribed to by all well-informed biologists as a fair statement of 

 the biological theory of nurture. Pearson has emphasised the 

 importance of the inherited nature, and we agree; our aim is to 

 emphasise the importance of nurture as well. 



I. It must be noted first of all that since both are indispensable 

 there is no antithesis. As one of the leaders of the experimental study 

 of heredity — Prof. T. H. Morgan — has said: a "character is the 

 product of a number of genetic factors and of environmental condi- 

 tions"; or, again, "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." 



We have admitted that the strength of an (inherited) individuality 

 may be such that it expresses itself almost in the face of inappro- 

 priate nurture ; but there is a minimum nurture necessary if there is 

 to be 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 differentiation. 



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. 



Fruit-flies (Drosophila) of a Mendelian race with a peculiar abnor- 

 mality may appear perfectly normal if raised in a dry bottle, but the 

 presence within them of the hereditary factor for "abnormal" may 

 be demonstrated by rearing their offspring in a wet bottle. 



A striking illustration of the present point concerns the red 

 Chinese primrose {Primula sinensis rubra). Reared at 15-20° it has 

 red flowers; reared at 30-35° C, with moisture and shade, the same 

 individual plants have pure white flowers. The development, so far 

 as colour goes, depends on its nurture. 



Take another illustration from the Fruit-fly. There is a mutant 

 stock that produces supernumerary legs, a considerable 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 flies with supernumerary legs occurs. In a hot climate 

 there would be no evidence that the peculiarity was part of their 

 inheritance; in a cold region it would be obvious. This shows that 

 the expression of the inheritance as regards a particular character 

 sometimes depends on nurture. 



