As introductory to examples of these must first, however, be named an example of such co-operation between the two great classes of vegetal organisms--the fungoid and the algoid. Incredible as the statement once seemed, it is a statement now accepted, that what we know as lichens, and used to consider as plants forming a certain low class, are now found to be not plants in the ordinary sense at all, but compound growths formed of minute algæ and minute fungi, carrying on their lives together: the algæ furnishing to the fungi certain constituents they need but cannot directly obtain, and the fungi profiting by certain materials they obtain from the algæ, either while living or while individually decaying. Whence it would seem that after the microscopic vegetal 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, enabling 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 analogous 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 amœbæ, made green by contained chlorophyll, continue 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 _Rhizopods_, various _Heliozoa_, numerous ciliated and flagellated _Infusoria_. Among _Metazoa_ the like association occurs in one of the sponges, in the _Hydra viridis_, in various turbellarians, 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 themselves where the light falls upon them, and can therefore further the production of the carbo-hydrates which eventually become useful to the animal cells, while these in some way reciprocate the benefit.