2l8 APPENDIX II INTENSIVE SEGREGATION. 



as well adapted to the environment of the other as to its own envi- 

 ronment. We may look upon the more important parts of a language 

 as persisting through their usefulness, the survival of the fittest being 

 the law ; but the divergent evolution which brings several languages 

 out of one seems to be principally due to other principles which are 

 closely akin to the principles that produce divergence in the organic 

 world. The fundamental condition in both organic and linguistic diver- 

 gence is isolation; and, this being secured, diversity of habits, bring- 

 ing diversity of aptitudes and diversity in the forms of survival, is sure to 

 arise even when the environment is the same. 



(4) Specific differences are not always differences of adaptation to the 

 environment, and those that are not should not be attributed to the action 

 of natural selection. It is admitted by every one that a distinction 

 relating to a character that is of no use in the economy of the organism 

 can not have arisen under the influence of natural selection. Those 

 who maintain that all specific distinctions are due to natural selection 

 maintain at the same time that these distinctions are adaptational 

 and advantageous. There are naturalists who maintain that the 

 very essence of the Darwinian theory is ' ' that specific differences must 

 be advantageous,"* and, therefore, adaptational, while they do not 

 claim the same for generic, family, and ordinate distinctions, or, indeed, 

 for varietal distinctions, if I rightly understand. I have never seen 

 any attempt to explain this supposed exception in the midst of the 

 taxonomic series ; and it seems to me that the break in the continuity 

 of nature which this interpretation of the Darwinian theory supposes 

 should lead us to a very careful investigation of the facts before we 

 accept it as a true interpretation of nature. 



I shall content myself with pointing out one distinction, occasion- 

 ally occurring between allied species, for which no use has ever been, 

 or is likely to be, found. I refer to the distinction between what are 

 known as dextral and sinistral forms. This distinction relates to the 

 form of the twisting of the animal and its shell. It is most easily 

 recognized by holding the shell with the aperture toward you with 

 the apex turned upward, and observing whether the aperture lies on 

 the right side of the central columella of the shell or on the left. 

 In the first case it is described as dextral, in the second as sinistral. 

 Inmost families and genera of water mollusks the sinistral form occurs 

 only as a sport (as in man the heart is sometimes found on the right 

 side) , and even among air-breathing mollusks the dextral form vastly 

 predominates. Of the Achatinellidae, Amastra and Leptachatina, 



* See letter from Mr. W. T. Thiselton Dyer, in Nature, vol. xxxix. p. 8 



