Our course of inquiry being thus in great measure determined by the present state of knowledge, we are compelled to follow an order widely different from this ideal one. It will be necessary first to give an account of those empirical generalizations which naturalists and physiologists have established: arranging them rather with a view to facility of comprehension than to logical sequence; and appending to those which admit of it, such deductive interpretations as First Principles furnish us with. Having done this, we shall be the better prepared for dealing with the leading truths of Biology, in connexion with the doctrine of Evolution.
\' VI PART II.
\ i CHAPTER T.
§ 43. Perhaps the widest and most familiar induction of Biology, is that organisms grow. While, however, this is a characteristic so habitually and markedly displayed by plants and animals, as to be carelessly thought peculiar to them, it is really not so. Under appropriate conditions, increase of size takes place in inorganic aggregates, as well as in organic aggregates. Crystals grow; and often far more rapidly than living bodies. Where the requisite materials are supplied in the requisite forms, growth may be witnessed in non-crystalline masses: instance the fungus-like accumulation of carbon that takes place on the wick of an unsnufFed candle. On an immensely larger scale, we have growth in geologic formations: the slow accumulation of deposited sediment into a stratum, is not distinguishable from growth in its widest acceptation. And if we go back to the genesis of celestial bodies, assuming them to have arisen by Evolution, these, too, must have gradually passed into their concrete shapes through processes of growth. Growth is indeed a concomitant of Evolution; and if Evolution of one kind or other is universal, growth is universal — universal, that is, in the sense that all aggregates display it in some way at some period.
The essential community of nature between organic growth and inorganic growth, is, however, most clearly seen on observing that they both result in the same way. The segregation of different kinds of detritus from each other, as well as from the water carrying them, and their aggregation into distinct strata, is but an instance of a universal tendency towards the union of like units and the parting of unlike units (First Principles, § 123). The deposit of a crystal from a solution, is a differentiation of the previously mixed atoms; and an integration of one class of atoms into a solid body, and the other class into a liquid solvent. Is not the growth of an organism a substantially similar process? Around a plant there exist certain elements that are like the elements which form its substance; and its increase of size is effected by continually integrating these surrounding like elements with itself. Nor does the animal fundamentally differ in this respect from the plant or the crystal. Its food is a portion of the environing matter, that contains some compound atoms like some of the compound atoms constituting its tissues; and either through simple imbibition or through digestion, the animal eventually integrates with itself, units like those of which it is built up, and leaves behind the unlike units. To prevent misconception, it may be well to point out that growth, as here defined, must be distinguished from certain apparent and real augmentations of bulk which simulate it. Thus, the long, white potatoshoots thrown out in the dark, are produced at the expense of the substances which the tuber contains: they illustrate not the accumulation of organic matter, but simply its rearrangement. Certain animal- embryos, again, during their early stages, increase considerably in size without assimilating any solids from the environment; and they do this by | absorbing the surrounding water. Even in the highest organisms, as in children, there appears sometimes to occur ^ a rapid gain in dimensions, that does not truly measure the added quantity of organic matter; but is in part due to changes analogous to those just named. Alterations of this riROwrir. 109 IvincI must not be confounded with that growth, properly so called, of which we have here to treat.
The next general fact to be noted respecting organic growth, is, that it has limits. Here there appears to be a distinction between organic and inorganic growth; but this distinction is by no means definite. Though that aggregation of inanimate matter which simple attraction produces, may go on without end; yet there appears to be an end to that more definite kind of aggregation which results from polar attraction. Different elements and compounds, habitually form crystals more or less unlike in their sizes; and each seems to have a size that is not usually exceeded without a tendency arising to form new crystals rather than, to increase the old. On looking at the organic kingdom as a whole, we see that the limits between which growth ranges, are very wide apart. At the one extreme, we have monads so minute as to be rendered but imperfectly visible by microscopes of the highest power; and at the other extreme, w^e have trees of 300 feet high, and animals of 100 feet long. It is true that though in one sense this contrast may be legitimately drawn, yet in another sense it may not; since these largest organisms are made by the combination of units that are individual^ like the smallest. A single plant of the genus Protococcus, is of the same structure as one of the many cells united together to form the thallus of some higher Alga, or the leaf of a phaenogam. Each separate shoot of a phgenogam is usually the bearer of many leaves. And a tree is an assemblage of numerous united shoots. One of these great teleophytes is thus an aggregate of aggregates of aggregates of units, which severally resemble protophytes in their sizes and structures; and a like building up is traceable throughout a considerable part of the animal kingdom. Even, however, when we bear in mind this qualification, and make our comparisons between organisms of the same degree of composition, we still find the limit of growth to have a great range. The smallest branched flowering plant is extremely insignificant by the side of a forest tree; and there is an enormious difference in bulk between the least and the greatest mammal. But on comparing members of the same species, we discover the limit of growth to be much less variable. Among the Protozoa and Protophyta, each kind has a tolerably constant adult size; and among the most complex organisms, the differences between those of the same kind that have reached maturity, are usually not very great. The compound plants do, indeed, sometimes present marked contrasts between stunted and well- grown individuals; but the higher animals diverge but inconsiderably from the average standards of their species.
On surveying the facts with a view of empirically generalizing the causes of these differences, we are soon made aware that by variously combining and conflicting with each other, these causes produce great irregularities of result. It becomes manifest that no one of them can be traced to its. consequences, unqualified by the rest. Hence the several statements contained in the following paragraphs, must be taken as subject to mutual modification.
h Let us consider first, the connexion between degree of growth and complexity of structure. This connexion being involved with many others, becomes apparent only on so averaging the comparisons, as to eliminate differences among the rest. Nor does it hold at all where the conditions are radically dissimilar; as between plants and animals. But bearing in mind these qualifications, we shall see that organization has a determining influence on increase of mass. Of plants the lowest, classed as Thallogens, usually attain no considerable size. Lichens, Algoe, and Fungi, count among their numbers bu.t few bulky species: the largest, such as certain Algae found in antartic seas, not serving greatly to raise the average. Though among Acrogens there are some, as the Tree-ferns, which attain a GUOWTir. Ill considerable height, the majority are but of humble growth.
The Endogens, including at one extreme small grasses and at the other tall palms, show us an average and a maximum greater than that reached by the Acrogens. And the Endogens are exceeded by the Exogens; among which are found the monarchs of the vegetal kingdom. Passing to animals, we meet the fact that the size attained by Vcrtehrata is usually much greater than the size attained by Invertehrata. Of invertebrate animals the smallest, classed as Protozoa^ are also the simplest; and the largest, belonorinff to the Annulosa and Molluscay are amono^ the most complex of their respective types. Of vertebrate animals we see that the greatest are Mammals; and that though, in past epochs, there were reptiles of vast bulk, their bulk did not equal that of the whale. Between reptiles and birds, and between land -vertebrates and aquatic vertebrates, the relation does not hold: the conditions of existence being in these cases widely different. But among fishes as a class, and among reptiles as a class, it is observable that, speaking generally, the larger species are framed on the higher tj^pes. The critical reader, who has mentally checked these statements in passing them, has doubtless already seen that this relation is not a dependence of organization on growth, but a dependence of growth on organization. The majority of Exogens are smaller than some Endogens; many Endogens are exceeded in size by certain Acrogens; and even among Thallogens, the least developed of plants, there are kinds of a size which many plants of the highest order do not reach. Similarly among animals: there are plenty of Crustaceans less than Adimce; numerous reptiles are smaller than some fish; the majority of mammals are inferior in bulk to the largest reptiles; and in the contrast between a mouse and a well- grown Medusa, we see a creature that is elevated in the scale of organization, exceeded in mass by one that is extremely degraded. Clearly then, it cannot be held that high organization is habitually accompanied by great size. The proposition here illustrated is the converse one, that great size is habitually accompanied by high organization. The conspicuous fact that the largest species of both animals and vegetals belong to the highest classes; and that throughout their various sub-classes the higher usually contain the more bulky forms; shows this connexion as clearly as we can expect it to be shown, amid so many modifying causes and conditions.
The relation between growth and supply of available nutriment, is too familiar a relation to need proving. There are, however, some aspects of it that must be contemplated before its implications can be fully appreciated. Among plants, which are all constantly in contact with the gaseous, liquid, and solid matters to be incorporated with their tissues; and which, in the same locality, receive not very unlike amounts of light and heat; diiferences in the supplies of available nutriment, have but a subordinate connexion with differences of growth. Though in a cluster of herbs springing up from the seeds let fall by a parent, the greater size of some than of others is doubtless due to better nutrition, consequent on accidental advantages; yet no such interpretation can be given of the contrast in size between these herbs and an adjacent tree. Other conditions here come into play: one of the most important probably being, an absence in the one case, and presence in the other, of an ability to secrete such a quantity of ligneous fibre as will produce a stem capable of supporting a large growth. Among animals, however, which (excepting some Entozoa) diifer from plants in this, that instead of bathing their surfaces, the matters they subsist on are dispersed, and have to be obtained; the relation between available food and growth, is shown with more regularity. The Protozoa, living on microscopic fragments of organic matter contained in the surrounding water, are unable, during their brief lives, to accumulate any considerable quantity of nutriment. Polypes and MoUuscoida, having for food these scarcely visible mem- J bers of the animal kingflom, are, tlioiigli large compared with their prey, small as measured by other standards: even, when aggregated into groups of many individuals, which severally catch food for the common weal, they are often so inconspicuous as readily to be passed over by the unobservant. And if from this point upwards we survey the successive grades of animals, it becomes manifest that, iii proportion as the size is great, the masses of nutriment are either large, or, what is practically the same thing, are so abundant and so grouped as that large quantities may be readily taken in. Thougli, for example, the greatest of mammals, the arctic whale, feeds on such comparatively small creatures as the acalephes and molluscs floating in the seas it inhabits, its method of gulping in whole shoals of them and filtering away the accompanying water, enables it to secure great quantities of food. We may then, with safety saj, that, other things equal, the growth of an animal depends on the abundance and sizes of the masses of nutriment which its powers enable it to appropriate. Perhaps it may be needful to add that, in interpreting this statement, the number of competitors must be taken into account. Clearly, not the absolute, but the relative, abundance of fit food is the point; and this relative abundance very much depends on how many individuals are competing for the food. Thus all who have had experience of fishing in Highland lochs, know that where the trout are numerous they are small, and that where they are comparatively large they are comparatively few.
What is the relation between growth and expenditure of force? is a question which next presents itself. Though there is reason to believe such a relation exists, it is not very readily traced: involved as it is with so many other relations. Some contrasts, however, may be pointed out, that appear to give evidence of it. Passing over the vegetal kingdom, throughout which the expenditure of force is too small to allow of such a relation being visible; let us seek in the animal kingdom, some case where classes otherwise allied, are contrasted in their locomotive activities. Let us compare birds on the one hand, with reptiles and mammals on the other. It is an accepted doctrine that birds are organized on a type closely allied to the reptilian tj^pe, but superior to it; and though in many respects the organization of birds is inferior to that of mammals, yet in other respects, as in the greater heterogeneity and integration of the skeleton, the more complex development of the respiratory system, and the higher temperature of the blood, it may be held that birds stand above mammals. Hence were growth dependent only on organization, we might infer that the limit of growth among birds should not be much short of that among mammals; and that the bird- type should admit of a larger growth than the reptile-type. Again, we see no manifest disadvantages under which birds labour in obtaining food, but from which reptiles and mammals are free. On the contrary, birds are able to get at food that is fixed beyond the reach of reptiles and mammals; and can catch food that is too swift of movement to be ordinarily caught by reptiles and mammals. Nevertheless, the limit of growth in birds, falls far below that reached by reptiles and mammals. With what other contrast between these classes, is this contrast connected? May we not suspect that it is connected with the contrast between their amounts of locomotive exertion?
Whereas mammals (excepting bats, which are small), are during all their movenients supported by solid surfaces or dense liquids; and whereas reptiles (excepting the ancient pterodactyles, which were not very large), are similarly restricted in their spheres of movement; the majority of birds move more or less habitually through a rare medium, in which they cannot support themselves without relatively great efforts. The conclusion that there exists this inverse ratio between growth and expenditure of force, is enforced by the significant fact, that those members of the class Aves, as the Dinornis and JEpiornis, which approached in J?ize to the larger Mammalia and Hrpfilia, were creatures incapable of fliglit — creatures which did not expend this excess of force in locomotion. Further evidence that there is an antagonism between the increase of bulk and the quantity of motion evolved by an organism, is supplied by the general experience, that human beings and domestic animals, when overworked while growing, are prevented from attaining the ordinary dimensions.
One other general truth concerning degrees of growth, must be set down. It is a rule, having exceptions of no great importance, that large organisms commence their separate existences as masses of organic matter more or less considerable in size, and commonly with organizations more or less advanced; and that throughout each organic sub-kingdom, there is a certain general, though irregidar, relation between the initial and the final bulks. Yegetals exhibit this relation much less clearly and constantly than animals. Yet though, among the plants that begin life as minute spores, there are some which, under their special conditions, grow to considerable sizes, the immense majority of them remain small. While, conversely, the great Endogens and Exogens, when thrown off from their parents, have already the formed organs of young plants, to which are attached large stores of highly nutritive matter. That is to say, where the young plant consists merely of a centre of development, the ultimate growth is commonly insignificant; but where the growth is to become great, there exists to start with, a well-developed embryo and a stock of assimilable matter. Throughout the animal kingdom, this relation is tolerably regular. Save among classes that escape the ordinary requirements of animal life, small germs or eggs do not give rise to bulky creatures. Where great bulk is to be reached, the young proceeds from an egg of considerable bulk, or is born of considerable bulk ready-organized and partially active. In the class fishes, for instance, a certain average proportion obtains between the sizes of the ova and the sizes of the adult indi- IIG THE INDUCTIONS OF BIOLOGY.
dividuals; and among the highest fishes, as sharks, the eggs are comparatively few and comparatively large. Reptiles have eggs that are smaller in number, and relatively greater in mass, than those of fishes; and throughout this class, too, there is a general ratio between the bulk of the egg and the bulk of the adult creature. As a group, birds show us a further limitation in the number of their eggs, and a further increase in their relative sizes; and from the minute eggs of the humming-bird up to the immense ones of the EpiorniSf holding several quarts, we see that, speaking generally, the greater the eggs, the greater the birds. Finally, among mammals (omitting the marsupials) the young are born, not only of comparatively large sizes, but with advanced organizations; and throughout this sub-division of the vertebrata, as throughout the others, there is a manifest connexion between the sizes at birth and the sizes at maturity. As having a kindred meaning, there must finally be noted the fact, that the young of these highest animals, besides starting in life with bodies of considerable sizes, almost full}^ organized, are, during subsequent periods of greater or less length, supplied with nutriment— in birds by feeding, and in mammals by suckling and afterwards by feeding. That is to say, beyond the mass and organization directly bequeathed, a bird or mammal obtains a further large mass at but little cost to itself Were an exhaustive treatment of the topic intended, it would be needful to give a paragraph to each of the many incidental circumstances by which growth may be aided or restricted. Such facts as that an entozoon is limited by the size of the creature, or even the organ, in which it thrives; that an epizoon, though getting abundant nutriment without appreciable exertion, is restricted to that small bulk at which it escapes ready detection by the animal it infests; that sometimes, as in the weazel, smallness is a condition to successful pursuit of the animals preyed upon; and that at other times, the advantage of resembling certain other crea- I tures, and so deceiving enemies or prey, becomes an indirect cause of restricted size. But the present purpose is simply to set down those most general relations between growth and other organic phenomena, which induction leads us to. Having done this, let us go on to inquire whether these general relations can be deductively established.
§ 44. That there must exist a certain dependence of growth on organization, may be shown a priori. When we consider the phenomena of Life, either by themselves or in their relations to surrounding phenomena, we see that, other things equal, the larger the aggregate the greater is the needful complexity of structure.
In plants, even of the highest type, there is a comparatively small mutual dependence of parts: a gathered flower-bud will unfold and flourish for days, if its stem be immersed in water; and a shoot cut off from its parent-tree and stuck in the ground, will grow. The respective parts having vital activities that are not widely unlike, it is possible for great bulk to be reached without that structural complexity required for combining the actions of parts. Even here, however, we see that for the attainment of great bulk, there requires such a degree of organization as shall co-ordinate the functions of roots and branches — we see that such a size as is reached by trees, is not possible without an efficient vascular system enabling the remote organs to utilize each other's products. And we see that such a co-existence of large growth with low organization, as occurs in some of the marine A/gce, occurs where the conditions of existence do not necessitate any considerable mutual dependence of parts — where the near approach of the plant to its medium in specific gravity, precludes the need of a well -developed stem, and where all the materials of growth being derived from the water by each portion of the thalhis, there requires no apparatus for transferring materials from part to part. Among animals w^hich, with but few 118 THE INDUCriONS OF BIOLOGY.
exceptions, are, by tlie conditions of their existence, required to take in nutriment through one specialized part of the body, it is clear that there must be a means whereby other parts of the body, to be supported by this nutriment, must have it conveyed to them. It is clear that for an equally efficient maintenance of their nutrition, the parts of a large mass must have a more elaborate propelling and conducting apparatus; and that in proportion as these parts undergo greater waste, a yet higher development of the vascular system is necessitated. Similarly with the pre-requisites to those mechanical motions which animals are required to perform. The parts of a mass cannot be made to move, and have their movements so co-ordinated as to produce locomotive and other actions, without certain structural arrangements; and, other things equal, a given amount of such activitj?" requires more involved structural arrangements in a large mass than in a small one. There must at least be a co-ordinating apparatus presenting greater contrasts in its central and peripheral parts.
The qualified dependence of growth on organization, is equally implied when we study it in connexion with that adjustment of inner to outer relations which constitutes Life. In plants this is not conspicuous, because the adjustment of inner to outer relations is but small. Still, it is visible in the fact that the condition on which only a plant can grow to a great size, is, that it shall, by the development of a massive trunk, present inner relations of forces fitted to counterbalance those outer relations of forces, which tend continually and occasionally to overthrow it; and this formation of a core of regularly- arranged woody fibres, is an advance in organization. Throughout the animal kingdom, this connexion of phenomena is manifest. To obtain materials for growth; to avoid injuries, which interfere with growth; and to escape those enemies which bring growth to a sudden end; implies in the organism, the means of fitting its movements to meet numerous external co-existences and sequences — implies sucli various structural arrangements as shall make possible tliese variously-adapted actions. It cannot be questioned that, everything else remaining constant, a more complex animal, capable of adjusting its conduct to a greater number of surrounding contingences, will be the better able to secure food and evade damage, and so to increase bulk. And evidently, without any qualification, we may say that a large animal, living under such complex conditions of existence as everywhere obtain, is not possible without xiomparatively high organization.
^yiiile, then, this relation is traversed and obscured by sundry other relations, it cannot but exist. Deductively we see that it must be modified, as inductively we saw that it is modified, by the circumstances amid which each kind of organism is placed; but that it is always a factor in determining the result.
§ 45. That growth is, cwtensjjaribus, dependent on the supply of assimilable matter, is a proposition so continually illustrated by special experience, as well as so obvious from general experience, that it would scarcely need stating, were it not requisite to notice the qualifications with which it must be taken.
The materials which each organism requires for building itself up, are not of one kind, but of several kinds. As a vehicle for transferring matter through their structures, all organisms require water as well as solid constituents; and however abundant the solid constituents, there can be no growth in the absence of water. Among the solids supplied, there must be a proportion ranging within certain limits. A plant round which carbonic acid, water, and ammonia exist in the right quantities, may yet be arrested in its growth by a deficiency of silica. The total absence of lime from its food, may stop the formation of a mammal's skeleton: thus dwarfing, if not eventually destroying, the mammal; and this, no matter what quantities of other needful colloids and crystalloids are furnished.
Again, the triitli that, other things equal, growth varies according to the supply of nutriment, has to be qualified by the condition, that the supply shall not exceed the ability to appropriate it. In the vegetal kingdom, the assimilating surface being external, and admitting of rapid expansion by the formation of new roots, shoots, and leaves, the effect of this limitation is not conspicuous: by artificially supplying plants with those materials which they have usuall}^ the most difiiculty in obtaining, we can greatly facilitate their growth; and so can produce striking differences of size in the same species. Even here, however, the effect is confined within the limits of the ability to appropriate; since in the absence of that solar light and heat, by the help of which the chief appropriation is carried on, the additional materials of growth are useless. In the animal kingdom this restriction is rigorous. The absorbent surface being, in the great majority of cases, internal; having a comparatively small area, which cannot be greatly enlarged without reconstruction of the whole body; and being in connexion with a vascular sj^stem, which must also be re- constructed before any considerable increase of nutriment can be made available; it is clear that beyond a certain point, very soon reached, increase of nutriment will not cause increase of growth. On the contrary, if the quantity of nutriment taken in, is greatly beyond the absorbent power, the excess, becoming an obstacle to the regular working of the organism, may retard growth rather than advance it.
While then it is certain, a priori, that there cannot be growth in the absence of such substances as those of which an organism consists; and while it is equally certain that the amount of growth must primarily be governed by the supply of these substances; it is not less certain that extra supply will not produce extra growth, beyond a point very soon reached. Deduction shows to be necessary, as induction makes familiar, the truths that, the value of food for purposes of growth depends not on the quantity of the various organizable materials it contains, but on the quantity of the material most needed; that given a right proportion of materials, the pre-existing structure of the organism limits their availability; and that the higher the structure, the sooner is this limit reached.