Speculation aside, it suffices to say that early in the history of life there must have arisen the distinction between _Protozoa_ 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 nitrogenous matters; and that the implied general co-operation serves in an indirect way to unite their lives, and in that sense to integrate the two kingdoms; needs not to be insisted upon. Further complications of the mutual dependence will be mentioned by and by. For the present it suffices to recognize this division of organic functions as the first which arose and as continuing to be that fundamental one which more than all others binds organisms at large together. § 314_c_. It will be thought by many readers that in speaking of the contrasted vital activities of plants and animals as constituting a “division of organic functions,” I am straining words beyond their meanings; since the conception of organic functions postulates an organized whole in which they exist, and plants and animals constitute no such organized whole. But there is at hand an unexpected defence for this conception--a defence not forthcoming a generation ago, but which now all biologists will recognize as relevant. I refer to the phenomena of symbiosis. These present various cases in which the plant-function and the animal-function are carried on in the same body,--cases in which the co-operation is not between separate vegetal organisms which accumulate nutritive matters and separate animal organisms which consume them, but is a co-operation between vegetal elements and animal elements forming parts of the same organism.