194 ANIMALS AND PLANTS vi 



progress where, the line between the animal and 

 the plant must be drawn. 



There is reason to think that certain organisms 

 which pass through a monad stage of existence, 

 such as the Myxomycetes, are, at one time of their 

 lives, dependent upon external sources for their 

 protein matter, or are animals; and, at another 

 period, manufacture it, or are plants. And seeing 

 that the whole progress of modern investigation is 

 in favour of the doctrine of continuity, it is a fair 

 and probable speculation though only a specu- 

 lation that, as there are some plants which can 

 manufacture protein out of such apparently in- 

 tractable mineral matters as carbonic acid, water, 

 nitrate of ammonia, metallic and earthy salts ; while 

 others need to be supplied with their carbon and 

 nitrogen in the somewhat less raw form of tartrate 

 of ammonia and allied compounds ; so there may be 

 yet others, as is possibly the case with the true 

 parasitic plants, which can only manage to put 

 together materials still better prepared still more 

 nearly approximated to protein until we arrive at 

 such organisms as the Psorospermice and the Pan- 

 histophyton, which are as much animal as vegetable 

 in structure, but are animal in their dependence 

 on other organisms for their food. 



The singular circumstance observed by Meyer, 

 that the Torula of yeast, though an indubitable 

 plant, still flourishes most vigorously when supplied 

 with the complex nitrogenous substance, pepsin ; 



