120 J. LIONEL TAYLER [august 



the fact that the particular variation in question must have been too 

 small to be by itself of selection value. In many cases the particular 

 variation would, no doubt, if taken alone be, as the objector asserts, 

 too unimportant to be selected, but as it is the whole organism that is 

 selected, it is not logical to make an artificial separation and study the 

 development of one organ or structure irrespective of the other organs 

 with which it is in nature associated. Every organ in its evolution 

 must be considered in relation to the whole of the particular organism 

 in which that particular stage of development of that organ is found. 

 Starting therefore with this fact that the net value of adaptability of 

 the whole organism to its environment must be the basis which deter- 

 mines selection or elimination, it will follow that certain lines of 

 development will result from the application of this criterion. In a 

 series of organisms placed under new conditions, elimination will pro- 

 ceed along lines essential to bring about a proper adjustment to the 

 new conditions. If the offspring of these adjusted organisms merely 

 repeated in their generation the characters of the exterminated as well 

 as of the surviving organisms, that temporary adjustment would be 

 permanent as long as the conditions were unchanged. But since the 

 offspring are produced only by the surviving organisms, selection is 

 continually raised to higher and higher planes of adaptation, and 

 therefore, as long as conditions remain constant, the tendency of 

 selection must be, as Darwin clearly saw, cumulative. He did not, 

 however, apparently see that from this cumulative tendency definite 

 variability must arise out of indefinite. 



Selection in direct relation to climatic conditions is therefore of 

 very minor importance, while selection among the members of a 

 species and all forms of inter-organismal selection is of infinitely more 

 importance, since it is this interaction, produced by the offspring in 

 different degrees inheriting the advantages of both parents (both of 

 whom have survived on account of certain advantages), that leads to 

 the cumulative development and never-ending struggle for survival. 

 Darwin came very near to this conception of definite variability when 

 he pointed out that " if a country were changing the altered conditions 

 would tend to cause variation, not but what I believe most beings vary 

 at all times enough for selection to act on." Extermination would 

 expose the remainder to " the mutual action of a different set of 

 inhabitants, which I believe to be more important to the life of each 

 being than mere climate," l and as " the same spot will support more 

 life if occupied by very diverse forms," l it is evident that selection 

 will favour very great diversity of structure. 



Bearing in mind this cumulative action of selection it will follow 

 that under constant or relatively constant conditions the struggle for 

 successful living will become more and more selective in character, 



*o 



1 From Poulton's " Charles Darwin and the Theory of Natural Selection " (Abstract of 

 Darwin's letter to Professor Asa Gray). 



