vi ANIMALS AND PLANTS 181 



and Diysdale of the existence of a contractile 

 vacuole in this monad, though they describe it in 

 another. 



Their Heteromita, however, multiplied rapidly 

 by fission. Sometimes a transverse constriction 

 appeared ; the hinder half developed a new cilium, 

 and the hinder cilium gradually split from its 

 base to its free end, until it was divided into 

 two ; a process which, considering the fact that 

 this fine filament cannot be much more than 

 i o 0*0 o o of an inch in diameter, is wonderful 

 enough. The constriction of the body extended 

 inwards until the two portions were united by a 

 narrow isthmus ; finally, they separated and each 

 swam away by itself, a complete Hetcromita, 

 provided with its two cilia. Sometimes the con- 

 striction took a longitudinal direction, with the 

 same ultimate result. In each case the process 

 occupied not more than six or seven minutes. 

 At this rate, a single Heteromita would give rise 

 to a thousand like itself in the course of an hour, 

 to about a million in two hours, and to a number 

 greater than the generally assumed number of 

 human beings now living in the world in three 

 hours ; or, if we give each Heteromita an hour's 

 enjoyment of individual existence, the same result 

 will be obtained in about a day. The apparent 

 suddenness of the appearance of multitudes of 

 such organisms as these, in any nutritive fluid to 

 which one obtains access, is thus easily explained 



