Every act of locomotion implies the expenditure of certain internal mechanical forces, adapted in amounts and directions to balance or outbalance certain external ones. The recognition of an object is impossible without a harmony between the changes constituting perception and particular properties coexisting in the environment. Escape from enemies supposes motions within the organism related in kind and rapidity to motions without it. Destruction of prey requires a particular combination of subjective actions, fitted in degree and succession to overcome a group of objective ones.
The difference of this correspondence in inanimate and animate bodies may be expressed by symbols. Let A be a change in the environment; and B some resulting change in an inorganic mass. Then A having produced B, the action ceases. But take a sufficiently organised living body, and let the change A impress on it some change C; then, while the environment A is occasioning a, in the living body, C will be occasioning c: of which a and c will show a certain concord in time, place, or intensity. And while it is in the continuous production of such concords or correspondences that life consists, it is by the continuous production of them that life is maintained.
As, in all cases, we may consider the external phenomena as simply in relation, and the internal phenomena also as simply in relation, the broadest and most complete definition of life will be:—the continuous adjustment of internal relations to external relations. It will be best, however, commonly to employ its more concrete equivalent—to consider the internal relations as "definite combinations of simultaneous and successive changes"; the external relations as "coexistences and sequences," and the connection between them as a "correspondence."
The Degree of Life Varies as the Degree of Correspondence
It is now to be remarked that the life is high in proportion as this correspondence between internal and external relations is well-fulfilled.
Each step upward must consist in adding to the previously adjusted relations which the organism exhibits some further relation, parallel to a further relation in the environment. And the greater correspondence thus established must, other things being equal, show itself both in greater complexity of life and greater length of life—a truth which will be duly realised on remembering the enormous mortality which prevails among lowly-organized creatures, and the gradual increase of longevity and diminution of fertility which is met with in ascending to creatures of higher and higher development. Those relations in the environment to which relations in the organism must correspond increase in number and intensity as the life assumes a higher form. Perfect correspondence would be perfect life.
Growth, or Increase of Bulk
Perhaps the widest and most familiar induction of biology is that organisms grow. Under appropriate conditions increase of size takes place in inorganic aggregates as well as in organic aggregates. Crystals grow. Growth is indeed a concomitant of evolution. The several conditions by which the phenomena of organic growth are governed, conspiring and conflicting in endless ways and degrees, qualify more or less differently each others' effects. Hence the following generalisations must be taken as true on the average, or other things equal:—
First, that growth being an integration with the organism of such environing matters as are of like nature with the matters composing the organism, its growth is dependent on the available supply of such matters. Second, that the available supply of assimilable matters being the same, and other conditions not dissimilar, the degree of growth varies according to the surplus of nutrition over expenditure. Third, that in the same organism the surplus of nutrition over expenditure is a variable quantity; and that growth is unlimited or has a definite limit according as the surplus does or does not progressively decrease,—a proposition exemplified by the increasing growth of organisms that do not expend force, and by the definitely limited growth of organisms that expend much force. Fourth, that among organisms that are large expenders of force, the size ultimately attained is, other things equal, determined by the initial size. Fifth, that where the likeness of other circumstances permits a comparison, the possible degree of growth depends upon the degree of organisation: an inference testified to by the larger forms among the various divisions and subdivisions of organisms.
Why Do Organisms Cease to Grow