in ON ANIMAL LIFE 111 



swimming more transitory, in the Medusae, on 

 the contrary, the fixed condition is apparently 

 only a phase in the production of the free 

 swimming animal. In both the one and the 

 other, however, the egg gives rise not to one 

 but to many mature animals. Steenstrup has 

 given to these curious phenomena, many other 

 cases of which occur among the lower animals, 

 and to which he first called attention, the 

 name of alternations of generations. 



In the life-history of Infusoria (so called 

 because they swarm in most animal or vege- 

 table infusions) similar difficulties encounter 

 us. The little creatures, many of which are 

 round or oval in form, from time to time 

 become constricted in the middle ; the con- 

 striction becomes deeper and deeper, and at 

 length the two halves twist themselves apart 

 and swim away. In this case, therefore, there 

 was one, and there are now two exactly sim- 

 ilar ; but are these two individuals ? They 

 are not parent and offspring that is clear, 

 for they are of the same age ; nor are they 

 twins, for there is no parent. As already 

 mentioned, we regard the Caterpillar, Chrys- 



