224 THE EFFECT OF TEMPERATURE 



any given variation is of blastogenic or somatogenic 

 origin; frequently, indeed, it is partly the one and 

 partly the other. Hence the term " variation " is con- 

 venient for general use, when one wishes to imply 

 nothing as to the origin of the observed condition, 

 whilst the narrower meaning may be applied to it when 

 it is mentioned in connection with the recognised and 

 closely defined term " modification." 



In our discussion of somatic modifications, it is found 

 most convenient to classify them according to the 

 agencies which have produced them, and not according 

 to the effects produced. Those observations in which 

 there appears to be a clear and direct relation between 

 cause and effect, between particular environment and 

 particular modification, will first be mentioned, and the 

 mixed and often indirect effects of more complex con- 

 ditions, such as climate and general environment, 

 studied later. 



Temperature. Of all the environmental conditions, 

 temperature is probably one of the most unequivocal in 

 its direct production of variations. That is to say, in 

 the majority of cases it acts directly on all the cells 

 composing the tissues, promoting or retarding their 

 growth, and producing a permanent effect on the organ- 

 ism as a whole. Of course it does not necessarily fol- 

 low that, because the rate of growth is altered, a per- 

 manent effect is produced on the absolute size of the 

 organism. Yet in the light of the observations de- 

 scribed in the last chapter, we are justified in assuming 

 that such is as a rule the case in animals. For plants it 

 will probably be generally admitted, if the difference 

 in the size of trees and plants grown in the tropics and 



