318 cook 



The introduction of a new term is always to be deprecated, 

 and may help very little, after all, in the explanation of a new 

 distinction. The word has to be explained, as well as the idea. 

 Nevertheless, there are occasions like the present, where progress 

 in expression is likely to be permanently hampered unless we 

 can be permitted to place definite labels upon our phenomena 

 and refer to them by unequivocal word-symbols. 



Symbasis, more properly than any other ascertained fact, 

 can be called a cause of evolution. It may not cause variation, 

 but it does enable variations to be combined into a general 

 evolutionary change of type. 



UTILITY OF NEW CHARACTERS. 



New characters, as mere fortuitous variations, might or might 

 not be useful, but if selection were the only cause of evolution, 

 progress would be limited to characters of definite utility. 

 Every character, therefore, which has attained to any consider- 

 able degree of expression would have a definite use, or would 

 have had use at some former time in the evolution of the 

 species. This logical necessity of predicating the utility of all 

 characters is the most obvious weakness of the theory of selec- 

 tion, for there are large numbers of character differences be- 

 tween species which are not only obviously useless at present 

 but which were probably equally useless in the past. 



Gulick's isolation theory does not insist on the utility of 

 specific differences, nor do the mutations of De Vries or the de- 

 terminate changes of Nageli and Weismann follow, of neces- 

 sity, the course of environmental utility. Selection would 

 explain the disappearance of types too far lacking in fitness, 

 but adaptation would remain a mere coincidence, depending on 

 whether adaptive variations happen to appear. 



Under the kinetic theory it is possible to admit that useful 

 and useless characters have equal possibilities of appearing and 

 evolving, as long as they do not become actually detrimental, 

 but at the same time selection is admitted to have a definite and 

 practical evolutionary function, since the rejection of harmful 

 tendencies has the power of enforcing more rapid specialization 

 in useful directions. Selection is, indeed, more effective for 



