126 CLAUDE W. MITCHELL 



transition may thus occur in a single generation and in the 

 embryo, just as we have found the sudden effects of nutrition to 

 occur in Asplanchna. On the other hand, the changes in external 

 conditions assumed as the cause are slow and gradual. This 

 continuity of the assumed cause may not preclude the possi- 

 bility of the production of discontinuous effects; but we think it 

 leads one to question their probability. 



In the article of Langhans ('06) in which the morphological 

 . characteristics of A. preodonta alone are considered, the in- 

 fluence of the viscosity of the water is mentioned along with 

 certain other important factors. He states (pp. 461 and 462) : 

 ''Es haben sich bei der Asplanchna preodonta grosse Varia- 

 tionen in der Grosse der Tiere gezeigt, " and 



Die Erscheinung der temporalen Grossenvariation wird bedingt durch 

 das Eintreten des quantitativen Maximums und ist weder auf eine 

 Anpassmig an die mit der Temperatur wechselnde Dichte des Wassers, 

 noch auf eine Anpassung an die mit der Temperatur wechselnde innere 

 Reibung des Wassers zuriickznfiihren. Sie dient also nicht der Regu- 

 lierung der Schwebefahigkeit, sondern ist eine direkte Folge der mehr 

 oder minder giinstigen Nahrungsverhaltnisse. 



Langhans' conclusions, though theoretical, are thus more 

 or less in general agreement with our own. They, however, 

 need experimental confirmation and we would conjecture that 

 especially the progressive changes observed will be found to 

 be due, not merely to 'giinstige Nahrungsverhaltnisse,' but to 

 sudden qualitative changes in food supply. 



Tn thus emphasizing the value of the nutritive factor and laying 

 weight" upon the outcome of our own experiments we do not 

 wish to belittle the possibility that other factors — possibly 

 primary, very probably secondary — may be experimentally 

 discovered which produce in rotifers, either fluctuating varia- 

 tion or mutational change. 



Meanwhile, the demonstration of a single definite primary 

 factor is of value in itself and offers, we think, a useful instrument 

 for further research along the lines of variation, sex-determina- 

 tion, and heredity. 



