100 Life and Death, Heredity and Evolution 



the animals could form spores which resisted a much higher 

 temperature (242) than that to which the active animals 

 were finally accustomed. In transforming from active ani- 

 mals to spores, the protoplasm must go through some 

 proces's which makes it more resistant to heat. It seems 

 probable that during the acclimatization the protoplasm 

 of the active animals went through a similar process. It 

 has been suggested that the essential point in both cases is 

 the getting rid of a certain proportion of the water in the 

 protoplasm, leaving it denser, for protoplasm containing lit- 

 tle water is as a rule less injured by heat than when it 

 contains much water. This new physical condition of the 

 protoplasm must then have persisted through reproduction, 

 and so been handed on to the offspring. 



Some effects of temperature in altering a different mani- 

 festation of the hereditary constitution have recently been 

 studied by Middleton (1918). Progeny of a given indi- 

 vidual of the infusorian Stylonychia pustulata were divided 

 into two sets; one set was kept at a high temperature, the 

 other at* a low temperature. Those at the high tempera- 

 ture (about 30 C.) divided more rapidly than those at 

 the low temperature (about 10 C.). After various inter- 

 vals, members of the two sets were brought to a common 

 intermediate temperature, and their rates of fission com- 

 pared. 



It was found that the stay in diverse temperatures had 

 altered the hereditary constitution so as to give diverse 

 rates of fission in the two stocks. After about thirty days 

 in the different temperatures, the set that had been kept 

 at high temperatures continued to divide more rapidly than 

 the others, even though both were now at the same tempera- 

 ture. But after longer periods in the diverse temperatures, 

 after two or three months or more, there was a change 



