TRANSITIONS IN MORPHOLOGICAL CHARACTERS 97 



It will be seen that a transition to the humped type did occur 

 in some of these mass cultures. This indicates the possibility 

 that a tendency toward the mutation-like change is inherent in 

 all individuals, although the presence of some unfavorable condi- 

 tion or the absence of some necessary stimulus may in some way 

 suppress it. But the discovery of such conditions or such stimulus 

 is not easy, for the factors that suggest themselves — the motor 

 stimulus due to the mere influence of numbers, sociality, so to 

 speak; the accumulation of metabolic products; or the recurrent 

 rhythm of transformation after the birth of a given number of 

 generations, from the resting egg — these and all other obvious 

 factors were essentially alike in all the cultures, yet the results 

 are at variance. 



In the hope of finding a clue to some external or internal factor, 

 especially of the nature of a physiological rhythm, plots were 

 made of the series A and D. The number of generations was used 

 as the abscissa and the number of offspring in each generation 

 was taken as the ordinate. The resultant curve* is shown in 

 chart 1. 



This curve does demonstrate a marked rhythm in reproduction. 

 Both series A and D show cycles which are in the main indepen- 

 dent of each other and therefore also independent of external 

 conditions. Both, however, show secondary curves, which seem- 

 ingly are related and which therefore probably are the result of 

 external conditions. The influence of these external conditions 

 appear definite though they last only for a short time. It is 

 hardly probable that these influences can be responsible for the 

 larger and more pronounced cyclic changes. These must b e 

 due rather to internal factors. They are evidently parallel to the 

 metabolic pulses or physiological rhythm which Calkins and others 

 have recorded for certain Protozoa. 



Assuming, then, certain periods or generations in which the 

 rotifer is in a higher physiological state than others, it is natural 

 to predict that this rhythm will be one of the factors to cause the 

 transition from the saccate to the humped type. However, 

 these isolation cultures prove conclusively that this factor is not 

 adequate in itself .to produce the transition. 



THE JOIRNAL OF EXPERIMENTAL ZOOLOGY, VOL. 15, NO. 1 



