3IO Experimental Zoology 



ence in the sea. This jellyfish produces eggs, and the ^gg after 

 fertiHzation develops into the primary polyp, completing the 

 cycle. The cycle is passed through during the summer, as a 

 rule, in our northern waters, and there is nothing to indicate 

 that external factors determine that a bud, developing later 

 in the season, must become a jellyfish and not a polyp. We may 

 suppose that differences in temperature or in the food supply 

 bring about the change, but as yet there is no proof that this 

 is the case. It would certainly be worth while to attempt to 

 find out whether external conditions are factors in the result; 

 but so far as we can see at present the jellyfish bud appears 

 when the colony reaches a certain size. It should not be over- 

 looked that this size is far from being fixed, and may vary in 

 different colonies when the jellyfish are formed. The impor- 

 tant consideration is that as the colony grows larger its relation 

 to the environment must become different from what it was at 

 first, so that even if the environment remains the same the animal 

 enters into a different relation with it. 



This point is well illustrated by certain recent experiments of 

 Klebs. The flowering plants produce at first only leaves and 

 branches. When they reach a certain size they produce flowers. 

 Klebs points out that most botanists look upon the flowering 

 of the plant as the culmination of its form. The form is some- 

 thing that perfects itself under favorable conditions without re- 

 spect to the environment. Klebs, on the other hand, thinks 

 that the development of the flowers is simply due to a relation 

 that becomes estabhshed between the plant (when it attains a 

 certain size or stage) and the external conditions, and he brings 

 forward in support of his idea a number of confirmatory ex- 

 periments. These show how by altering the conditions a 

 shoot that would ordinarily develop into a flowering branch 

 continues to grow vegetatively, producing only leaves. 



In the green hydra the change from the non-sexual to the 

 sexual mode of reproduction seems to be connected with defi- 

 nite seasons of the year, and possibly, therefore, with a change 

 in the environment. Throughout the summer, the autumn, and 



