TRANSITIONS IN MORPHOLOGICAL CHARACTERS 121 



the process of species-making. It also tends decidedly to justify 

 the application of the concept of mutation to the sudden changes 

 which have been here described. 



It needs hardly to be pointed out after the above experiments, 

 that a change in food organism or a qualitative nutritional differ- 

 ence constitutes the major factor which is instrumental in pro- 

 ducing the sudden changes. All other factors turn out negative, 

 or at least negligible in results, as compared to food change. 



The most important factor is plainly a qualitative change in 

 diet; yet it seems probable that a quantitative change, a mere 

 alternation of hunger with high feeding, is an effective though 

 less potent factor. Thus it was observed that in mass cultures, 

 fed on Paramecium in which the mutation took place, the food 

 supplied was quickly eaten, the digestive tracts of the rotifers 

 again becoming empty before the next meal. A similar condition 

 occurs as a matter of course in feeding upon the larger organisms, 

 Hydatina, Brachionus, and Moina. A single such organism 

 furnishes a large and nutritious meal for the Asplancha and the 

 digestion of this meal is by no means always followed immediately 

 by another. In other words, the feeding upon these large and 

 active organisms inevitably means an irregular food supply in 

 which hunger or sheer starvation alternate in the most varied 

 ways with over-nutrition. This is strictly true with Moina feed- 

 ing, individuals in starvation and depletion always being found 

 in such cultures. It is unfortunate, however, that in our effort 

 at periodic feeding in isolation cultures no positive results were 

 secured. The truth is that such feeding simply does not succeed. 



In regard to mass cultures, however, it should be pointed out 

 that qualitative food change as a cause of mutation can never be 

 wholly excluded. This is because of the tendency towards at 

 least occasional cannibalism which the species always manifests, 

 the ingestion of a young Asplanchna by an older individual ob- 

 viously constituting a marked change of diet. This change of 

 diet has been assigned by Dr. Powers in the paper above men- 

 tioned as the cause of the second mutation, from the humped to 

 the campanulate type. 



