1 ON ANIMAL LIFE 111 
swimming more transitory, in the Medusz, 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- 
