VI ANIMALS AND PLANTS 187 



they are readily transported by the gentlest 

 breeze. Since, again, the zoospores set free from 

 each spore, in virtue of their powers of locomotion, 

 swiftly disperse themselves over the surface, it is 

 no wonder that the infection, 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 organ-' 

 ism, which, in one stage of its existence, is truly a 

 "Monad," indistinguishable by any important 

 character from our Heteromita, and extraordinarily 

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

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

 metamorphoses 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 conjugation takes place in the Perono- 

 spora. Two separate portions of its protoplasm 

 become fused together, surround 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 number of 

 zoospores like those already described, each of 

 which, after a period of activity, germinates in the 



