The term final causes refers to a series of phenomena that are met with only where there is life, and that tend toward a definite purpose or end. Living organisms take nutriment from their environment, to the end of assimilating it, that is, transforming it from an inert, indifferent substance into a substance that is a living part of themselves.
This phenomenon is undoubtedly one of the most characteristic. But there are still other forms of final cause, such for example as the transformation of the fertilised ovum into the fully developed individual, predetermined in its essential characteristics, such as form, dimensions, colour, activities, etc. There are ova that to all appearances are exactly alike; the human ovum itself is nothing more than a simple cell composed of protoplasm and nucleus, measuring only a tenth of a millimeter (= 1/250 inch); yet all these ovum cells produce living organisms of the utmost diversity; yet so definitely predetermined that, if we know to what species the ovum belongs, we are able to predict how many bones will compose the skeleton of the animal destined to develop from it, and whether this animal will fly or creep upon the ground, or rise to take a place among those who have made themselves the lords of the earth. Furthermore, knowing the phases of development, we may predetermine at what periods the successive transformations that lead step by step to the complete development of the individual will take place.
Another form of final cause is seen in the actions of living creatures, which reveal a self-consciousness; a consciousness that even in its most obscure forms guides them toward a destined end.
Thus, for example, even the infusoria that may be seen through a microscope in a drop of water, chasing hither and thither in great numbers, avoiding collision with one another, or contending over some particle of food, or rushing in a mass toward an unexpected ray of light, give us a keen impression of their possession of consciousness, a dim glimmering of self-will, which is the most elementary form of that phenomenon that manifests itself more and more clearly, from the metazoa upward, through the whole zoologic scale: the final cause of psychic action.
Again, in multicellular organisms there are certain continuous and so-called vital phenomena, which some physiologists attribute to cellular consciousness: for example, the leucocytes in the blood seem to obey a sort of glimmering consciousness when they rush to the encounter of any danger threatening the organism, and ingest microbes or other substances foreign to the blood; and it is also due to a phenomenon that cannot be explained by the physical laws of osmosis, that the erythrocytes or red blood corpuscles and the plasma in the blood never interchange sodium salts for those of potassium; and lastly the cells of each separate gland seem to select from the blood the special substances that are needed for the formation of their specific products: saliva, milk, the pancreatic juice, etc.
Still another manifestation of final cause is the tendency exhibited by each living individual to make a constant struggle for life, a struggle that depends upon a minimum expenditure of force for a maximum realisation of life, thanks to which life multiplies, invades its environment, adapts itself to it, and is transformed.
Another fundamental synthetic characteristic of life is the limitation of form and size that is a fixed and constant factor in the characteristics of each species; the body of the living individual cannot grow indefinitely.
Living creatures do not increase in quantity by the successive accumulation of matter, as is the case with inorganic bodies, but by reproduction, that is, the multiplication of individuals.
Through the phenomenon of reproduction, life has a share in the eternity of matter and of force, that is, in a universal phenomenon. But what distinguishes it is that the individual creatures produced by other living individuals form, each one of them, an indivisible element in which life manifests itself; and this element is morphologically fixed in the limits of its form and size.
The peculiarities which are attributed to the chemical action of protoplasm are of an analytic character, so far as they concern the fundamental characteristics of life. The constant interchange of matter, namely, metabolism, constitutes undoubtedly a phenomenon peculiar to living matter, protoplasm; but protoplasm does not exist apart from living organisms. And what constitutes its chief characteristic is that, when brought into contact with it, inert substances are assimilated, i.e., they become like it, or rather, are transformed into protoplasm; mineral salts such as the nitrates or nitrites of sodium and potassium are transformed in the case of plants into living plasma capable of germinating either into a rose bush or a plane tree or a palm, and inert organic substances such as bread or wine are transformed into human flesh and blood. So that the phenomenon of assimilation outweighs, as a characteristic of life, the molecular chemical action through which it is accomplished. Since metabolism does not occur in nature as a chemical phenomenon, and cannot be produced artificially, but is found only in the matter composing living organisms, it follows that life is the cause of this form of dynamic action, and not that this dynamic action is the cause of life.[4]