Part 21

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

Why should not all organisms, when supplied with sufficient material, continue to grow as long as they live? We have found that organisms are mostly built up of compounds which are stores of force. These substances being at once the materials for organic growth and the sources of organic force, it follows, from the persistence of force, that growth is substantially equivalent to the absorbed nutriment minus the nutriment used up in action. This, however, does not account for the fact that in every domestic animal the increments of growth bear continually decreasing ratios to the mass, and finally come to an end. Nevertheless, it is demonstrable that the excess of absorbed over expended nutriment must decrease as the size increases. Since in similar bodies the areas vary as the squares of the dimensions and the masses vary as the cubes, it follows that, however great the excess of assimilation over waste may be during the early life of an active organism, there must be reached, if the organism lives long enough, a point at which the surplus assimilation is brought to nothing--a point at which expenditure balances nutrition, a state of moving equilibrium. Obviously, this antagonism between assimilation and expenditure must be a leading cause of the contrast in size between allied organisms that are in many respects similarly conditioned.

Development, or Increase of Structure

In each of the organic sub-kingdoms the change from an incoherent, indefinite homogeneity to a coherent definite heterogeneity is illustrated in a quadruple way. The originally-like units or cells become unlike, in various ways, and in ways more numerously marked as the development goes on. The several tissues which these several classes or cells form by aggregation, grow little by little distinct from each other; and little by little become structurally complex. In the shoot as in the limb, the external form, originally very simple and having much in common with countless simple forms, organic and inorganic, gradually acquires an increasing complexity, and an increasing unlikeness to other forms, and meanwhile, the remaining parts of the organism, having been developed severally, assuming structures diverging from each other and from that of this particular shoot or limb, there has arisen a greater heterogeneity in the organism as a whole.

The most remarkable induction of von Baer comes next in order. It is that in its earliest stage every organism has the greatest number of characters in common with all other organisms in their earliest stages; that at each subsequent stage traits are acquired which successively distinguish the developing embryo from groups of embryos that it previously resembled--thus step by step diminishing the group of embryos which it still resembles; and that thus the class of similar forms is finally narrowed to the species of which it is a member. For example, the human germ, primarily similar to all others, first differentiates from vegetal germs, then from invertebrate germs, and subsequently assumes the mammalian, placental unguiculate, and lastly the human characters.

The development of an individual organism is at the same time a differentiation of its parts from each other and a differentiation of the consolidated whole from the environment; and in the last as in the first respect there is a general analogy between the progression of an individual organism and the progression of the lowest orders of organisms to the highest orders.

The Laws of Multiplication

Every living aggregate being one of which the inner actions are adjusted to balance outer actions, it follows that the maintenance of its moving equilibrium depends on its exposure to the right amounts of these actions. Its moving equilibrium may be overturned if one of these actions is either too great or too small in amount: either by excess or defect of some inorganic or organic agency in its environment.

Our inquiry resolves itself into this:--in races that continue to exist what laws of numerical variation result from these variable conflicting forces?

The forces preservative of a race are two--ability in each member of the race to preserve itself, and ability to produce other members. These must vary inversely--one must decrease as the other increases. We have to ask in what way this adjustment comes about as a result of evolution.

Including under individuation all those processes completing and maintaining individual life, and under genesis all those aiding the formation and perfecting of new individuals, the two are necessarily antagonistic. Every higher degree of individual evolution is followed by a lower degree of race multiplication, and vice versâ. Progress in bulk, complexity or activity involves retrogress in fertility; and progress in fertility involves retrogress in bulk, complexity, or activity. The same quantity of matter may be divided into many small wholes or few large wholes; but number negatives largeness, and largeness negatives number.

It is a general physiological truth that while the building-up of the individual is going on rapidly, the reproductive organs remain imperfectly developed and inactive; and that the commencement of reproduction at once indicates a declining rate of growth and becomes a cause of arrest in growth.

It has now to be noticed how complexity of organisation is hindered by reproductive activity and conversely. The hydra's power to produce young ones from nearly all parts of its body is due to the comparative homogeneity of its body, while it is not improbable that the smallness of human fertility, compared with the fertility of large feline animals, is due to the greater complexity of the human organisation--more especially the organisation of the nervous system.

Of the inverse variation between activity and genesis we have examples in the contrast between the fertility of birds and the fertility of mammals. Comparing the large with the large and the small with the small, we see that creatures which continually go through the muscular exertion of sustaining themselves in the air and propelling themselves rapidly through it are less prolific than creatures of equal weights which go through the smaller exertion of moving about over solid surfaces. The extreme infertility of the bat is most striking when compared with the structurally similar but very prolific mouse; a difference in the rate of multiplication which may fairly be ascribed to the difference in the rate of expenditure.

Interpretation and Qualification

Derived as the self-sustaining and waste-sustaining forces are from a common stock of force, it necessarily happens that, other things being equal, increase of the one involves decrease of the other. It may therefore be set down as a law that every higher degree of organic evolution has for its concomitant a lower degree of the peculiar organic dissolution which is seen in the production of new organisms.

How is the ratio between individuation and genesis established in each case? All specialties of the reproductive process are due to the natural selection of favourable variations. Given a certain surplus available for race preservation, and it is clear that by indirect equilibration only can there be established that peculiar distribution of this surplus which is seen in each case.

Here a qualification must be made. Recognising the truth that every increase of evolution which is appropriate to the circumstances of an organism brings an advantage somewhat in excess of its cost, the general law, more strictly stated, is that genesis decreases not quite so fast as individuation increases. The result of greater individuation--whether it takes the form of greater strength or higher speed, facilitates some habitual movement or utilises better the absorbed aliment--is a greater surplus of vital capital; part of which goes to the aggrandisement of the individual and part to the formation of new individuals. Hence every type that is best adapted to its conditions has a rate of multiplication that insures a tendency to predominate. Survival of the fittest, acting alone, is ever replacing inferior species by superior species. But beyond the longer survival, and therefore greater chance of leaving offspring, which superiority gives, we see here another way in which the spread of the superior is insured. Though the more evolved organism is the less fertile absolutely, it is the more fertile relatively.

Multiplication of the Human Race

What causes increase or decrease of genesis in other creatures causes increase or decrease of genesis in man. It is true that, even more than hitherto, our reasonings are here beset with difficulties. So numerous are the inequalities in the conditions that but few unobjectionable comparisons can be made. The human races differ not only in their sizes and foods, and in the climates they inhabit, but also their expenditures in bodily and mental action are extremely unequal.