218 APPENDIX II—INTENSIVE SEGREGATION. 
as welladapted to the environment of the other as to its own envi- 
ronment. We may look uponthe moreimportant parts of alanguage 
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 ts 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 ts 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. Itis 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. 
In most familiesand 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 Achatinellide, Amastra and Leptachatina, 
* See letter from Mr. W. T. Thiselton Dyer, in Nature, vol. xxx1x, p. 8 
