HOMOLOGY AND ITS INTERPRETATION 53 



which offer instances of otherwise disparate and unrelated 

 organisnis that are inseparably bound together, in some appar- 

 ently capricious and fortuitous respect, by a preadaptation 

 of the one to the other. Parasites, guests, or symbiotes, as the 

 case may be, they are now indissolubly wedded to some de- 

 terminate species of host by reason of an appropriate and con- 

 genital adjustment. For all that, however, the association 

 seems to be a contingent one, and it appears incredible that 

 the associates were always united, as at present, by bonds of 

 reciprocal advantage, mutual dependence, or one-sided exploi- 

 tation. Yet the basis of the relationship is in each case a now 

 inherited adaptation, which, if it does not represent the primi- 

 tive condition of the race, must at some time have been 

 acquired. For phenomena such as these, orthogenesis, which 

 makes an organ the exclusive product of internal factors, con- 

 ceiving it as a preformed mechanism that subsequently se- 

 lects a suitable function, has no satisfactory explanation. 

 Lamarckism, which asserts the priority of function and makes 

 the environment mold the organ, is equally inacceptable, in 

 that it flouts experience and ignores the now demonstrated 

 existence of internal hereditary factors. But, if between these 

 two extremes some evolutionary via media could be found, 

 one must confess that it would offer the only conceivable 

 "natural explanation" of preadaptation.* All this, of course, is 

 pure speculation, but it serves to show that here, at any rate, 

 the theory of Transformism occupies a position from which it 

 cannot easily be dislodged. 



But, besides the advantage of being able to offer a "natural 

 explanation" of the association of homology with adaptation, 



* Vernon Kellogg has expressed this same view in a recent article, 

 though he frankly admits that it is an as yet unrealized desideratum. 

 "Altogether," he says, "it must be fairly confessed that evolutionists 

 would welcome the discovery of the actual possibility and the mechan- 

 ism of transferring into the heredity of organisms such adaptive 

 changes as can be acquired by individuals in their lifetime. It would 

 give them an explanation of evolution, especially of adaptation, much 

 more satisfactory than any other explanation at present claiming the 

 acceptance of biologists." (Atlantic Monthly, April, 1924, p. 488.) 



