8 
THE TASMANIAN NATURALIST 
But besides the large lakes in Tasmania the smallest ponds and 
puddles will be found to contain Entomostraca. 
These are all varieties of the three types figured below; they never 
run to any great size but are always visible to the naked eye. 
You may wonder how it is that these acquatic animals can survive 
and multiply in small pools which must frequently dry up in the heat of 
summer. The method by which this is effected in the Cladocera was 
discovered some years ago by the great German biologist and student of 
heredity, Professor Weisman. He found out that whilst favourable con¬ 
ditions prevail these animals were all female, which produced very 
rapidly eggs that did not require fertilisation in order to develop. But 
when unfavourable conditions arrived, such as extreme heat or cold, the 
females began to lay a different kind of egg, which had to be fertilized 
by the male, and which was afterwards encased in a little air-tight box 
made by the female out of her own shell. 
The fertilized egg, so shut up, is now capable of being frozen or 
dried up, or blown about by the wind, indeed almost anything except 
burnt, and still when it falls into a favourable pond or puddle, after an 
interval of even several years, it will hatch out and give rise to another 
generation of water fleas. 
Weisman also believed that the production of these ‘resting’ eggs 
was not directly called forth by the approach of danger, but that each 
different kind of water flea produced them at regular intervals mechanic- 
