310 Experimental Z oology 
ence in the sea. This jellyfish produces eggs, and the egg after 
fertilization 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 established 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 
