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THE AMERICAN NATURALIST [Vol. LIII 



SO far as they may be regarded as scientific theories 

 at all. 



The theory of mutation, in its original form, postulated 

 large and abrupt variations as the material for selection, 

 a modification which does not affect the principle essen- 

 tially. In its later form, it merely insists that these 

 variations must be of the discontinuous or Mendelian 

 type, assuming that all other variations are non-inheri- 

 table. Those who maintain the importance of isolation 

 in evolution can not, of course, regard this as a vera 

 causa of adaptive change. The actual changes must be 

 either ''spontaneous" variations or mutations or else 

 modifications due to environment. Thus, we must resort 

 finally to either the Lamarckian or the Darwinian prin- 

 ei]^le to account for such of them as prove to be useful. 

 "Orthogenesis," so far as it is not a vague appeal to a 

 "perfecting principle, " "elan vital" or the like, is a mere 

 assertion that variations may accumulate in a given 

 direction independently of selection. Wherever the vari- 

 ations are sufficiently adaptive, however, we are not justi- 

 fied in excluding selection. When non-adaptive, such a 

 process presents no greater difficulty in principle than the 

 continuous growth of a crystal or the continuous deepen- 

 ing of a canyon by erosion. 



Most of us are prepared to admit that much in the 

 organic world is non-adaptive. We may even grant that 

 a large proportion of the diagnostic characters of species 

 and genera belong to this category. Such characters, 

 while they may baffle the investigator, are in general not 

 such as would have suggested the operation of a super- 

 natural factor in evolution. In this paper we are con- 

 cerned with the problem of organic adaptation, and shall 

 leave aside the origin of characters which are useless to 

 the organism. 



In fhe foregoing analysis, I have regarded adaptive 

 response, ivhether of structnre or fiinciiou, as hf'nig in- 

 variably a secondary p}ii'}n)}}i<'})(>u . Tin' cmi th'dini) be- 

 tween the need of the orgaiusin mid the nicius ad,-'^iinif>- 

 to satisfy this is believed to have ahcays been, at the out- 



