210 THE BIOLOGY OF THE SEASONS 



reimpressed on successive generations of individuals, and 

 a number of evolutionists (Mark Baldwin, Lloyd Morgan, 

 Osborn, and James Ward) have pointed out that modifica- 

 tions may thus serve as screens until coincident variations 

 (similar in appearance, though intrinsic not extrinsic in 

 origin, and with a greater grip) can emerge and establish 

 themselves. 



Suppose, for a moment, that there was an island where 

 swarthiness of skin became, through climatic changes, 

 absolutely essential to success in life. Suppose that a large 

 proportion of the population became so tanned that they 

 met the critical test and survived survived in virtue of 

 plasticity or modifiability. They would b.e in great 

 difficulties in regard to their children, awaiting the anxious 

 day when it would become plain that their children were 

 also plastic or modifiable in the direction of sun-burning. 

 Suppose, however, that meanwhile there was a widespread 

 constitutional tendency to swarthiness, suppose that more 

 and more innately swarthy children were born, does it 

 not seem extremely probable that the unentailed, purely 

 modificational swarthiness might act as a protective 

 screen, fostering the innate entailed swarthiness ? 



To sum up, then, in a statement that will apply to 

 ourselves as well as to the plants and animals around us : 

 The complex environment produces modifications, which 

 are often very important to the individual, and may also be 

 of indirect importance in the evolution of the race ; it also 

 supplies the appropriate nurture (in the widest sense), 

 without which the inheritance cannot express itself ; and 

 it probably affords though we know little of this as yet a 

 succession of stimuli, provoking some of those germinal 

 variations which form the raw material of organic progress. 



