398 PHYSIOLOGICAL DEVELOPMENT. 



in a shadowy way by their modern representatives. And the 

 implication is that during the period throughout which these 

 smallest, lowest, and simplest living things alone existed, 

 there could have been, in the absence of kinds, no mutual 

 dependence. 



Since, among various of the lowest types now known to 

 us, the same individual exhibits a life which is now pre 

 dominantly vegetal and now predominantly animal, we 

 cannot err in assuming that there eventually took place 

 differentiations of this original plant-animal type into types 

 permanently unlike : some in which the traits were more 

 markedly vegetal and others in which they were more 

 markedly animal. As fast as this differentiation arose, 

 there came the beginnings of cooperation between the pre 

 dominantly vegetal types which by the aid of light formed 

 organic matter from the inorganic world, and the predomi 

 nantly animal types which, in chief measure, utilized the 

 matter so formed. Evidently with the rise of such a dif 

 ferentiation came an incipient mutual dependence. If to 

 the implied algoid type and the animal type there be added 

 the fungoid type, somewhat intermediate in character, which 

 in a large proportion of cases lives on the decaying remnants 

 of the other two, we are furnished with a rude conception of 

 the primary differentiations and the accompanying vague 

 mutual dependences. 



Speculation a side, it suffices to say that early in the history 

 of life there must have arisen the distinction between Pro 

 tozoa and Protophyta, and that this distinction foreshadowed 

 that widest contrast which the higher organic world presents 

 the contrast between plants and animals. It is needless 

 to do more than name the mutual dependence between these 

 two great divisions. That, as being respectively decomposers 

 of carbon dioxide and exhalers of carbon dioxide, they act 

 reciprocally, as also in some measure by interchange of nitro 

 genous matter? ; and that the implied general cooperation 

 serves in an indirect way to unite their lives, and in that 



