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THE AMERICAN NATURALIST [Vol. XL VI 



of the two forms separated by the gap. The greatness of our 

 lack of a series connecting any vertebrate with any invertebrate 

 whatever, must impress one more and more the more he knows 

 and thinks about the problem. The only other conceivable mode 

 of demonstration would be to induce, experimentally, the trans- 

 formation of some living invertebrate into an undoubted verte- 

 brate. The possibility of accomplishing such a fact is so slight 

 that no biologist is likely to try it. 



This brings us to a point where the transcendent importance 

 comes to view, of the logic of the interpretation of evolution, 

 understanding by evolution the transformation of one kind, or 

 species, of organism into another kind, or species. Except in 

 the relatively simple and rare cases of proof through direct ob- 

 servation, experimental or other, the fact that all evidence rests 

 back absolutely on resemblance — on similarity in form and com- 

 position of the parts of organisms — is of the utmost importance. 

 No evolutionist hesitates in either word or intention to accept 

 this tenet ; yet when it comes to the actual ascertainment of like- 

 nesses and differences, and to speculating on their phylogenefie 

 significance, the danger of shifting from the inductive to the 

 deductive mode of reasoning, and of going down before the fal- 

 lacy known in the logic books as petitio principii, or surreptitious 

 assu»ij>(i<»>, is so imminent and subtle that very few of us, even 

 those most wide awake for pitfalls, avoid it wholly. Almost if 

 not quite all the numberless hypothetical ancestral organisms 

 that have been summoned to the aid of speculation on descent 

 daring the last half century and more, are, I am convinced, vic- 

 tims of this evil, some in greater, some in less degree. The fal- 

 lacious reasoning usually runs something like this: Assuming 

 such-or-such an animal once existed, it is easy to see how a par- 

 ticular organ or part of the animal actually before us may have 

 arisen from that ancestry. Says Patten : 



^\ e have merely to strip off the superficial disguise of our hypothetical 



their mode of growth, . . . does or does not harmonize with the assump- 

 tion that they are the ancestors of the vertebrates (p. 3). 



Before this "mere" stripping off begins, would it not be well 

 to consider "our hypothetical arachnid ancestors " rather closely? 

 // such an animal actually once existed, let it be granted for the 

 moment that it might have undergone such a transformation as 

 is conjectured. Surely the first point to be pressed is as to the 



