EVOLUTION IN NATURE 147 



organism is so closely linked with a great array of 

 environmental conditions, any attempt to explain 

 its evolution as the result, primarily, of a single 

 one, partakes too strongly of the limitations which 

 have hitherto characterized evolutionary theories. 

 It is doubtful that any thoughtful biologist 

 would choose to defend a narrow view, but the 

 implications which this situation suggests to me 

 have been quite generally neglected. Indeed, I 

 know of less than a dozen writers who have ably 

 and persistently maintained a broad attitude to- 

 ward the problem of evolutionary change. Un- 

 less all permanent modification of the heritage 

 is due to radiation it must be associated with other 

 environmental factors, in the sense that the chro- 

 mosomes change in response to conditions surround- 

 ing them in the body which are directly or in- 

 directly associated, in turn, with conditions in the 

 external environment. This conclusion follows 

 logically from our knowledge of natural phenom- 

 ena. Given identical heritages, or identical genes 

 if you prefer, and identical environments for them, 

 and we can expect only identical results. The 

 environment fluctuates, not spontaneously but as 

 a result of the complex interaction of natural 

 forces, hence the expression of the heritage must 

 always differ as it responds to various conditions 

 of the environment. Therefore a change in a gene 

 must follow from some change in the complex 



