LIFE CYCLE OF HYDATINA SENTA 153 



exerted always in the same direction. A comparison of tables 18 

 and 19 with table 20 shows that this is not the case. A lower 

 temperature may result in either more or fewer male-producers. 

 The results of the several experiments are not contradictory if 

 we assume that the influence of temperature is indirect. Let us 

 suppose that all the conditions, whether external or internal, 

 with the exception of temperature, which exist at a given time, 

 tend to produce at a given temperature either a higher or a lower 

 proportion of male-producers than prevailed previous to that 

 time. If at a lower temperature the response of the rotifers to 

 other conditions is less than at higher temperatures, all the results 

 here obtained find their explanation. In tables 18 and 19 we 

 may suppose that conditions at room temperature tended to pro- 

 duce fewer male-producers; in the ice-chest, the rotifers did not 

 respond to these conditions to the extent that they did at room 

 temperature, and the result was more male-producers at the lower 

 temperature. In table 20, it seems that a set of conditions was 

 present at the end of January which tended to produce more male- 

 producers, and that the low temperature of the ice-chest caused 

 the rotifers to respond to these conditions to a smaller degree, the 

 result being fewer male-producers at the lower temperature. This 

 view encounters no difficulty in the fact that no difference in the 

 the proportion of male-producers was obtained at a temperature 

 of 20° and 24.^^5 C, respectively; for if the temperature is suffici- 

 ently high that the rotifers may respond to other conditions to 

 the greatest degree of which they are capable, it may make 

 little difference which of several moderately high temperatures 

 prevails. 



In offering this explanation I have assumed that the response 

 of the rotifers to both external and internal conditions may be 

 modified by temperature. It is obviously possible to assume that 

 the response to external conditions alone is so modified. In that 

 case, the results shown in table 20 demand a set of external con- 

 ditions which tended to increase the proportion of male-producers 

 above that usually obtained from the same line in spring water. 

 It is possible that such external agents exist. Whitney ('10) 

 believes that there are chemical substances having such an effect. 



