ASPECTS OF KINETIC EVOLUTION 203 



It is usually possible to discover somewhere between the pro- 

 hibitive extremes an optimum condition, or a locality where the 

 fullest development of the species takes place. Unfavorable 

 conditions multiply as the boundaries are approached, and 

 development is variously impeded and restricted, but surely the 

 ability of the organisms to accept or to avoid a measure of such 

 restrictions and to achieve an existence in spite of them, is small 

 warrant for concluding that the conditions afford an adequate 

 biological explanation of the characters. Still less are we justi- 

 fied in supposing that the unfavorable peripheral conditions are 

 any more truly causative than the central optima. Adverse cir- 

 cumstances, by restricting development, would seem rather to 

 require the organism to put forth more active energies, not of 

 development merely, but of accommodation as well. And yet 

 it is in abnormal features arising under abnormal conditions 

 that the evidences of environmental causation have been chiefly 

 found. 



If each species w r ere restricted to an absolute uniformity of 

 conditions and materials, the doctrine of environmental causa- 

 tion would have had at least a partial justification, whereas the 

 versatility of organisms, instead of demonstrating environmental 

 causation, renders it highly improbable. The individual mem- 

 bers of species in nature are different, even under the same con- 

 ditions ; why should we expect them to be alike under different 

 conditions? 



For some species the range of environmental conditions is 

 very broad, in others very narrow. The fitness of the latter 

 type of species may appear to be greater than the former, in the 

 sense of being more highly specialized. It is not, however, the 

 extent of narrowly specialized fitness, but the extent of widely 

 varied adjustment which generally determines the range of dis- 

 tribution and the numerical prosperity of the species. 



In a general way the power of a species to accommodate 

 itself to different environments might be held to favor evolution, 

 because it would improve the chances of sustained numerical 

 prosperity, which is an evolutionary advantage. It does not 

 appear, however, that " plasticity" w r ould be especially helpful 

 in the evolution of the particular characters which might be 



