400 PHYSIOLOGICAL DEVELOPMENT. 



type had become in a large degree differentiated into two 

 main types, in adaptation to different conditions of life, and 

 had acquired appropriate specialities of nature, there grew up 

 this communistic arrangement between certain of them, en- 

 abling each to benefit by the powers which the other had 

 acquired: evidently an exchange of services, a physiological 

 division of labour, a mutual dependence -of functions analo- 

 gous to that which exists between functions in an ordinary 

 plant or animal. 



Not differing in principle but only in application, is that 

 symbiosis above referred to as existing between Protophyta 

 and many Protozoa, as well as between such Protophyta 

 and the lowest kinds of Metazoa. A recent statement that 

 certain amoebae, made green by contained chlorophyll, con- 

 tinue to grow and multiply after they have consumed what 

 nutritive matter may be at hand, is in harmony with various 

 facts alleged of other Protozoa various other kinds of Khizo- 

 pods, various Heliozoa, numerous ciliated and flagellated 

 Infusoria. Among Metazoa the like association occurs in 

 one of the sponges, in the Hydra viridis, in various turbel- 

 larians, in a rotifer, and even in two molluscs. In these 

 cases the partnership between the vegetal cells and the 

 animal cells (existing either as units or as an organized group 

 such as a polype), is a partnership which, as before, profits 

 each of the partners an inference supported by the fact that 

 Metazoa containing these algoid cells usually place them- 

 selves where the light falls upon them, and can therefore 

 further the production of the carbo-hydrates which event- 

 ually become useful to the animal-cells, while these in some 

 way reciprocate the benefit. 



Here, then, we have exchange of services between asso- 

 ciated plant-elements and animal-elements a performance 

 by them of different organic functions for the benefit of the 

 aggregate which they unite to form. Hence, when these 

 vegetal elements and animal elements are separately em- 

 bodied in plants and animals, which profit by one another, 



