ALTERNATION OF GENERATIONS 83 



getting back and fort', from one species of host to another, 

 as is so often necessary. For example, the liver fluke is 

 in the snail for a while, in the liver of the sheep a while, 

 and in the water of the pond between times. It is during 

 these alternations of generations, with their differences of 

 structure and habits and instincts, that these perilous 

 changes are made. 



