I I 8 PROTOZOA 



spores discharged by the rupture of the cyst (Fig. 38). These spores 

 grow from a size too minute for resolution by our microscopes 

 into the ordinary flagellate form. They withstand the effects of 

 drying, if this be effected immediately on their escape from the 

 ruptured cyst ; so that it is probable that each spore has itself 

 a delicate cyst-wall and an aplanospore, from which a single 

 zoospore escapes. The complex cycle, of course, comprises the 

 whole course from spore-formation to spore-formation. Such 

 complete and regular " life-histories," each characteristic of the 

 species, were the final argument against those who held to the 

 belief that spontaneous generation of living beings took place in 

 infusions of decomposing organic matter. 



Previous to the work of these observers it had been almost 

 universally believed that the temperature of boiling water was 

 adequate to kill all living germs, and that any life that appeared 

 in a closed vessel after boiling must be due to spontaneous change 

 in its contents. But they now showed that, while none of the 

 species studied resisted exposure in the active condition to 

 a temperature of 138°-140° F., the spores only succumbed, in 

 liquid, to temperatures that might even reach 268° F., or when 

 dry, even 300° F. or more. Such facts explain the constant 

 occurrence of one or more such minute species in liquids putre- 

 fying under ordinary conditions, the spores doubtless being 

 present in the dust of the air. Very often several species may co- 

 exist in one infusion ; but they separate themselves into different 

 zones, according to their respective need for air, when a drop of 

 the liquid is placed on the slide and covered for examination. 

 Dallinger ^ has made a series of experiments on the resistance of 

 these organisms in their successive cycles to a gradual rise of 

 temperature. Starting with a liquid containing three distinct 

 species, which grew and multiplied normally at 60" F., he placed 

 it under conditions in whicli he could slowly raise the tempera- 

 ture. While all the original inmates would have perished at 

 142° F., he succeeded in finally producing races that throve at 

 158° ¥., a scalding heat, when an accident put an end to that 

 series of experiments. In no instance was the temperature raised 

 so much as to kill off the beings, so that the increased tolerance of 

 their descendants was due not, as might have been anticipated, 

 to selection of those that best resisted, but to the inheritance of 



1 In P.S.S. xxvii. 1878, p. 332. 



