Organic Evolution. 239 



the perfect insect; if so, it may, perhaps, throw some 

 light on the melanism so often remarked in north-country 

 examples of widely distributed moths." Mr. Cockerell* 

 regards moisture as the determining condition of a certain 

 phase of melanism, especially among Lepidoptera. The 

 same author states that the snail " Helix nemoralis was 

 introduced from Europe into Lexington, Virginia, a few 

 years ago. Under the new conditions it varied more than 

 I have ever known it to do elsewhere, and up to the 

 present date (1890) 125 varieties have been discovered 

 there. Of these, no less than 67 are new, and unknown 

 in Europe, the native country of the species." The effects 

 of the salinity of the water on the brine-shrimp Artemia 

 have already been mentioned. One species with certain 

 characteristics was transformed into another species with 

 other characteristics by gradually altering the saltness of 

 the water. So, too, in the matter of food, the effects of 

 feeding the caterpillars of a Texan species of Saturnia on 

 a new food-plant were so marked that the moths which 

 emerged were reckoned by entomologists as a new species. 

 The point, I repeat, to be especially noted about these 

 cases and others which might be cited,f is that the varia- 

 tion produced is a definite variation. Very probably it is 

 generally, or perhaps always, produced in the embryonic 

 or larval period of life. In some cases the variation seems 

 to be transmissible, though definite and satisfactory proofs 

 of this are certainly wanting. Still, we may say that if 

 the changed conditions be maintained, the resulting varia- 

 tion will also be maintained. Under these conditions, at 

 least, the variation is a stable one. It is probable that, 

 apart from preferential mating, the varieties thus produced 

 will tend to breed together rather than to be crossed with 

 the parent form or varieties living under different con- 

 ditions. In this way varieties may sometimes arise by 



Nature, vol. xli. p. 393. 



t See Professor Meldola's edition of Professor Weismann's " Studies in 

 the Theory of Descent," and Mr. Cunningham's translation of Professor 

 Eimer's " Organic Evolution." 



