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 Fun- gi, 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 En- dogens 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, be- lonorinff 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 be- ing 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 or- ganization on growth, but a dependence of growth on or- ganization. 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 mam- mals 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, ex- ceeded in mass by one that is extremely degraded. Clearly then, it cannot be held that high organization is habitually 112 THE INDUCTIONS OF BIOLOGY.
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 be- fore 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 spring- ing 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 inter- pretation 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 se- crete 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 GROWTH. 113 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 compara- tively 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 rela- tions. 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 8 114 THE INDUCTIONS OF BIOLOGY.
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 de- pendent 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 mani- fest 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 re- stricted 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 GROWTH. 115 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 ge- neral experience, that human beings and domestic animals, when overworked while growing, are prevented from attain- ing 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 con- siderable 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. Rep- tiles 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 ge- nerally, 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 ad- vanced organizations; and throughout this sub-division of the vertebrata, as throughout the others, there is a mani- fest 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 sub- sequent periods of greater or less length, supplied with nutri- ment— 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 with- out 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 GROWTH. 117 I GROWTH. 117 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 com- paratively 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 pos- sible 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 locomo- tive and other actions, without certain structural arrange- ments; 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 counter- balance 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 — GROWTH. 119 GROWTH. 119 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 exist- ence as everywhere obtain, is not possible without xiompara- tively 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 or- ganism is placed; but that it is always a factor in determin- ing the result.
§ 45. That growth is, cwtensjjaribus, dependent on the sup- ply of assimilable matter, is a proposition so continually illus- trated by special experience, as well as so obvious from general experience, that it would scarcely need stating, were it not re- quisite 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 how- ever 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.
120 THE INDUCTIONS OF BIOLOGY.
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 re- construction 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 GROWTH. 121 GROWTH. 121 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.
§ 46. But why should the growth of every organism be finally arrested? Though the rate of increase may, in each case, be necessarily restricted within a narrow range of varia- tion— though the increment that is possible in a given time, cannot exceed a certain amount; yet why should the incre- ments decrease, and finally become insensible? Why should not all organisms, w^hen supplied with sufficient materials, continue to grow as long as they live? To find an answ^er to this question, we must first revert to the nature and functions of organic matter.
In the first three chapters of Part I., it was shown that plants and animals mainly consist of substances in states of unstable equilibrium — substances which have been raised to this unstable equilibrium by the expenditure of the forces we know as solar radiations, and which give out these forces in other forms, on falling into states of stable equilibrium. Leaving out the water, which serves as a vehicle for these materials and a medium for their changes; and excluding those mineral matters that play either passive or subsidiary parts; organisms are built up of compounds which arc stores of force. Those complex colloids and crystalloids which, as united together, form organized bodies, are the same colloids and crystalloids w^hich give out, on their decomposition, the forces expended by organized bodies. Thus these nitrogeneous and carbonaceous substances, being at once the materials for organic growth and the sources of organic force; it results that as much of them as is used up for the, genesis of force, is taken away from the means of growth; and as much as is economized by diminishing the genesis of force, is available for growth. Given that limited quantity 122 THE INDUCTIONS OF BIOLOGY.
of nutritive matter whicli tlie pre-existing structure of an organism enables it to absorb; and it is a necessary corollary from the persistence of force, that the matter accumulated as growth, cannot exceed that surplus which remains unde- composed, after the production of the required amounts of sensible and insensible motion. This, which would be rigorously true under all conditions, if exactly the same substances were used in exactly the same proportions, for the production of force and for the formation of tissue, requires, however, to be taken with the qualification, that some of the force-evolving substances are not constituents of tissue; and that thus, there may be a genesis of force which is not at the expense of potential growth. But since organisms (or at least animal organisms, with which we are here chiefly concerned,) have a certain power of selective absorption, which, partially in an individual and more completely in a race, adapts the proportions of the substances absorbed to the needs of the sj^'stem; then if a certain habitual expenditure of force, leads to a certain habitual absorption of force- evolving matters that are not available for growth; and if, were there less need for such matters, the ability to absorb matters available for growth would be increased to an equi- valent extent; it follows that the antagonism described, does, in the long run, hold even without this qualification. Hence, growth is substantially equivalent to the absorbed nutriment, minus the nutriment used up in action, This, however, is no answer to the question — why has individual growth a limit? The antagonism described, does not manifestly 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, other things equal, become less as the size of the animal becomes greater. In similarly-shaped bodies, the masses vary as the cubes of the dimensions; whereas the strengths vary as the squares of the dimensions. See here GROWTH. 123 the solution of the problem. Supposing a creature which a year ago was one foot high, has now become two feet high, while it is michangcd in proportions and structure; what are the necessary concomitant changes that have taken place in it? It is eight times as heavy; that is to say, it has to re- sist eight times the strain which gravitation puts on its structure; and in producing, as well as in arresting, every one of its movements, it has to overcome eight times the inertia. Meanwhile, the muscles and bones have sever- ally increased their contractile and resisting powers in pro- portion to the areas of their transverse sections; and hence are severally but four times as strong as they were. Thus, while the creature has doubled in height, and while its ability to overcome forces has quadrupled, the forces it has to overcome have grown eight times as great. Hence, to raise its body through a given space, its muscles have to be contracted with twice the intensity, at a double cost of matter expended. This necessity will be seen still more clearly if we leave out the motor apparatus, and consider only the forces required and the means of supplying them. For since, in similar bodies, the areas vary as the squares of the dimensions, and the masses vary as the cubes; it follows that the absorbing sur- face has become four times as great, while the w^eight to be moved by the matter absorbed has become eight times as great. If then, a year ago, the absorbing surface could take up twice as much ' nutriment as was needed for expenditure, thus leaving one-half for growth, it is now able only just to meet expenditure, and can provide nothing for growth. How- ever great the excess of assimilation over waste, may be dur- ing the early life of an active organism., we see that because a series of numbers increasing as the cubes, overtakes a series increasing as the squares, even though starting from a much smaller number, there must be reached, if the organism lives long enough, a point at which the surplus assimilation is brought down to nothing — a point at w^hich expenditure ba- lances nutrition — a state of moving equilibrium. This,