ANIMALS AND PLANTS. 



tion, once started, soon spreads from field to field, and 

 extends its ravages over a whole country. 



However, it does not enter into my present plan to 

 treat of the potato disease, instructively as its history 

 bears upon that of other epidemics ; and I have selected 

 the case of the Peronospora simply because it affords an 

 example of an organism, which, in one stage of its exist- 

 ence, is truly a " Monad," indistinguishable by any 

 important character from our Heteromita, and extraor- 

 dinarily like it in some respects. And yet this " Monad " 

 can be traced, step by step, through the series of meta- 

 morphoses which I have described, until it assumes the 

 features of an organism, which is as much a plant as is 

 an oak or an elm. 



Moreover, it would be possible to pursue the analogy 

 farther. Under certain circumstances, a process of con- 

 jugation takes place in the Peronospora. Two separate 

 portions of its protoplasm become fused together, sur- 

 round themselves with a thick coat, and give rise to a 

 sort of vegetable egg called an oospore. After a period 

 of rest, the contents of the oospore break up into a num- 

 ber of zoospores like those already described, each of 

 which, after a period of activity, germinates in the ordi- 

 nary way. This process obviously corresponds with the 

 conjugation and subsequent setting free of germs in the 

 Heteromita. 



But it may be said that the Peronospora is, after all, 

 a questionable sort of plant ; that it seems to be wanting 

 in the manufacturing power, selected as the main dis- 



