328 



THE AMERICAN NATURALIST 



[Vol. XL 



in answering this question in the affirmative, but he does beheve 

 that the illustrations which follow are suggestive and certainly 

 point to some such derivation. It is not held, even by de Vries, 

 I take it, that the mutation theory is to supplant or take the place 

 of the older evolution by whose slow and gradual processes 

 (natural selection, survival, environment, distribution, etc.) the 

 present state of animal and vegetal matter has been reached, but 

 as an additional process in that great scheme of life. 



In certain mollusks the species seem to be unstable, that is, 

 they have a tendency to vary, not in a given direction but in many 



directions at the same time. These seem to come under the head 

 of mutants, or sports. The fresh-water pulmonates belonging 

 to the genus liymniea are examples of this class ivnd every species 

 which has been studied in any quantity has Ix-cn found to \:iry 

 in this manner. Lymnoca 'paiustris Miiller f - chxh s \ i> one 

 of the most notably variable, and its mutations arc many and 

 marked. Fig. 1 re{)resents a set of ten shells of this si)ecies col- 

 lected by Mr. L. K. Daniels in Muskag swamps, Ilalma, Minne- 

 sota. They vary from a long, narrow shell, with elevated spire 



