SOURCES OF CHANGE 49 



that change may occur entirely from within in 

 some cases. But, paradoxically, physics and chem- 

 istry provide us at the same time with an abun- 

 dance of facts in the phenomena of the colloid state 

 which seem to dispense with the necessity for such 

 explanations of organic change. External condi- 

 tions are commonly the cause of chemical and 

 physical transformations, and the usual manifesta- 

 tions of life are inseparably linked with external 

 conditions. 



In seeking the cause of change, the variability of 

 the environment is so conspicuous that it is every- 

 where to be seen. The variation of the heritage is 

 no less obvious, since variation is a universal char- 

 acteristic of living things, and the determinative 

 powers exercised by the organism are apparently 

 functions dependent upon the nature of its herit- 

 age. 



Variability of the environment impinges upon 

 organisms in a number of ways. In a given locality 

 marked fluctuations may occur, and in any locality 

 some fluctuation is inevitable, due to the topog- 

 raphy of the land and the meteorological condi- 

 tions prevailing. High altitudes, for example, are 

 characterized by rarefied air. This factor not only 

 conditions the respiration of animals, but also in- 

 fluences temperature. During the day when the 

 sun shines it is a poor insulator, hence the body is 

 exposed to intense heat. I have picked caterpillars 



