No. 643] SEROLOGICAL PHENOMENA 



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perfecting principle, to merely a general trend in devel- 

 opment due to tlie natural constitutional restrictions of 

 the germinal materials, or to the physical limitations 

 imposed by a narrow environment. In most modern 

 statements of the theory, the idea of continuous and pro- 

 gressive change in one or more characters, due according 

 to some to internal factors, according to others to external 

 causes — evolution in a " straight line " — seems to be 

 the central idea. To many, faced by the seeming im- 

 possibility of explaining by natural selection the origins 

 of new characters, it has been apparently merely a wel- 

 come general utility concept by which one may account 

 for the beginnings of new organs, or the development of 

 parts along definite lines, irrespective of utility. 



For present purposes nothing is to be gained by a 

 review of the different theories of orthogenesis, all so 

 well summarized in Kellogg's ^' Darwinism To-day," and 

 I shall proceed merely on the assumption that, judging 

 from the statistical law of errors, certain variations are 

 apparently not fortuitous, since they tend to accumulate 

 in certain directions. It is customary to add that the 

 lines of development which result are independent of, and 

 in extreme cases may be opposed to, the operation of 

 natural selection. I see no reason, however, for believing 

 that if variations occur in definite directions of no use 

 to the organism, why they may not also occur just as 

 definitely in directions which lend themselves to the per- 

 fecting influences of natural selection. The difficulty in 

 determining this point lies in the fact that an evolution 

 based on the selection of even fortuitous variations must 

 in one sense be orthogenetic, that is, along definite lines, 

 so that there is no way in retrospect of telling whether 

 the underlying germinal variations were purely fortu 

 itous, or whether they were biased toward an adaptive 

 outcome. 



Of the various lines of evidence brought forward in 

 support of theories of orthogenesis, the ones which ap- 

 peal most convincingly to me are: (1) those based on 



