§ 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 variation— though the increment that is possible in a given time, cannot exceed a certain amount; yet why should the increments 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 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 undecomposed, 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 forceevolving 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 equivalent 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 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 resist 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 severally increased their contractile and resisting powers in proportion 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 surface 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. However great the excess of assimilation over waste, may be during 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 balances nutrition — a state of moving equilibrium. This, however, thougli the chief, is not the sole, varying relation between degrees of growth and amounts of expended force. There are two more; one of which conspires with the last, while the other conflicts with it. Consider in the first place, the cost at which nutriment is distributed through the body, and effete matters removed from it. Each increment of growth being added at the periphery of the organism, the force expended in the transfer of matter must increase in a rapid progression — a progression more rapid than that of the mass.
Bat as the dynamic expense of distribution is small compared with the dynamic value of the materials distributed, this item in the calculation is unimportant. Now consider, in the second place, the changing proportion between production and loss of heat. In similar organisms, the quantities of heat generated by similar actions going on throughout their substance, must increase as the masses, or as the cubes of the dimensions. Meanwhile, the surfaces from which loss of heat by radiation takes place, increase only as the squares of the dimensions. Though the loss of heat does not therefore increase only as the squares of the dimensions, it certainly increases at a smaller rate than the cubes. And to the extent that augmentation of mass results in a greater retention of heat, it effects an economization of force. This advantage is not, however, so important as at first appears. Organic heat is a concomitant of organic action, and is so abundantly produced during action, that the loss of it is then of no consequence: indeed the loss is often not rapid enough to keep the supply from rising to an inconvenient excess. It is onl}^ in respect of that maintenance of heat which is needful during quiescence, that large organisms have an advantage over small ones in this relatively diminished loss. Thus these two subsidiary relations between degrees of growth and amounts of expended force, being in antagonism with each other, we may conclude that their differential result does not greatly modify the result of the chief relation previously set forth.
Any one who proceeds to test this deduction, will find some seeming incongruities between it and certain facts inductively established. Lest these should mislead him, it will be well to explain them. Throughout the vegetal kingdom, he may remark that there is no limit of growth except what death entails. Passing over a large proportion of plants which never exceed a comparatively small size, because they wholly or partially die down at the end of the year; and pointing to trees that annually send forth new shoots, even when their trunks are hollowed out by decay; he may ask — How does growth happen here to be unlimited? The answer is, that plants are only accumulators; they are in no appreciable degree expenders. As they do not undergo a waste which increases as the cubes of the dimensions, while assimilation increases as their squares; there is no reason why their growth should be arrested by the equilibration of assimilation and waste. Again, should he look among animals for an exact correspondence between the decreasing increments of growth as ascertained by observation and as determined by deduction, he will not find it. And there are sufficient reasons why the correspondence cannot be more than approximate. Besides the fact above noted, that there are other varying relations which complicate the chief one, he must bear in mind that the bodies compared are not truly similar: the proportions of trunk to limbs and trunk to head, vary considerably. The comparison is still more seriously vitiated by the inconstant ratio between the constituents of which the body is composed. In the flesh of adult mammalia, water forms from 68 to 71 per cent., organic substance from 24 to 28 per cent., and inorganic substance from 3 to 5 per cent.; whereas in the foetal state, the water amounts to 87 per cent., and the solid organic constituents to only 1 1 per cent. Clearly this change from a state in which the force- evolving matter forms one tenth of the whole, to a state in which it forms two and a half tenths, must greatly interfere with the parallelism between the actual and the theoretical progression. Yet another difficulty may come under his notice. The crocodile 126 THE IXBUCTTONS OF BIOLOGY.
is said to grow as long as it lives; and there appears reason to think that some predaceous fishes, such as the pike, do the same. That these animals of comparatively high organization, have no definite limits of growth, is, however, an exceptional fact due to the exceptional non-fulfilment of those conditions which entail limitation. AYhat kind of life does a crocodile lead? It is a cold-blooded, or almost coldblooded, creature; that is, it expends very little for the maintenance of heat. It is habitually inert: not chasing prey, but lying in wait for it; and undergoes considerable exertion only during its occasional brief contests with prey. Such other exertion as is, at intervals, needful for moving from place to place, is rendered small by the small difi'erence between the animal's specific gravity and that of water. Thus the crocodile expends in muscular action, an amount of force that is insignificant compared with the force commonly expended by land-animals. Hence its habitual assimilation is diminished much less than usual by habitual waste; and beginning with an excessive disproportion between the two, it is quite possible for the one never quite to lose its advance over the other while life continues. On looking closer into such cases as this and that of the pike, which is similarly cold-blooded, similarly lies in wait, and is similarly able to obtain larger and larger kinds of prey as it increases in size; we discover a further reason for this absence of a definite limit. The mechanical causes necessitating a limit, are here only partially in action. For a creature living in a medium of nearly the same density as its body, has not constantly to overcome that gravitative force which is the chief resistance t) be met by terrestrial animals: it has not to expend for this purpose, a muscular power that is large at the outset, and increases as the cubes of its dimensions. The only force increasing as the cubes of its dimensions, which it has thus to overcome, is the inertia of its parts. The exceptional continuance of growth observed in creatures so circumstanced^ is therefore perfectly explicable.
f § 47. Obviously tliis antagonism between accumulation and expenditure, must be a leading cause of the contrasts in size between allied organisms that are in many respects similarly conditioned. The life followed by each kind of animal, is one involving a certain average amount of exertion for the obtainmcnt of a given amount of nutriment — an exertion, part of which goes to the gathering or catching of food, part to the tearing and mastication of it, and part to the afterprocesses requisite for separating the nutritive atoms — an exertion which therefore varies according as the food is abundant or scarce, fixed or moving, according as it is mechanically easy or difficult to deal with when secured, and according as it is, or is not, readily soluble. Hence, while among animals of the same species having the same mode of life, there will be a tolerably constant ratio between accumulation and expenditure, and therefore a tolerablj^ constant limit of growth; there is every reason to expect that different species, following different modes of life, will have unlike ratios between accumulation and expenditure, and therefore unlike limits of growth.
Though the facts as inductively established, show a general harmony with this deduction, we cannot usually trace this harmony in any specific way; since the conflicting and conspiring causes which affect growth are so numerous. The only contrast which seems fairly to the point, is the beforenamed one between the vertebrates which fly, and the most nearly- allied vertebrates which do not fly: the differences in degrees of organization and relations to food, being not such as seriously to affect the comparison. If it be admitted that birds habitually expend more force than mammals and reptiles, then it will follow a priori, that, other things being tolerably equal, they should have a lower limit of growth than mammals and reptiles; and this we know to be the fact a posteriori.
§ 48. One of the chief causes, if not the chief cause, of the differences between the sizes of organisms, has yet to be considered. We are introduced to it by pushing the above inquiry a little further. Small animals have been shown to possess an advantage over large ones, in the greater ratio which, other things equal, assimilation bears to expenditure; and we have seen that hence, small animals in becoming large ones, gradually lose that suj^plus of assimilative power which they had, and eventually cannot assimilate more than is required to balance waste. But how come these animals while young and small, to have surplus assimilative powers? Have all animals equal surplus of assimilative powers? And if not, how far do differences between the surpluses determine differences between the limits of growth? We shall find in the answers to these questions, the interpretation of many marked contrasts in growth that are not due to any of the causes above assigned. For example, an ox immensely exceeds a sheep in mass. Yet the two live from generation to generation in the same fields, eat the same grass and turnips, obtain these aliments witii the same small expenditure of force, and differ scarcely at all in their degrees of organization. Whence arises, then, their striking unlikeness of bulk?
We noted when studying the phenomena of growth inductively, that organisms of the larger and higher types, commence their separate existences, as masses of organic matter having tolerable magnitudes. Speaking generally, we saw that throughout each organic sub-kingdom, the acquirement of great bulk occurs only where the incipient bulk and organization are considerable; and that they are the more considerable in proportion to the complexity of the life which the organism is to lead.
The deductive interpretation of this induction may best be commenced by an analogy. A street orange-vendor makes but a trifling profit on each transaction; and unless more than ordinarily fortunate, he is unable to realize during the day a larger amount than will meet his wants: leaving him to start on the morrow in the same condition as before. The trade of the huxter in ounces of tea and halfpounds of sugar, is one similarly entailing much labour for small returns. Beginning with a capital of a few pounds, it is impossible for liim to have a shop large enough, or goods sufficiently abundant and various, to permit an extensive business: he must be content with the half-pence and pence which he makes by little sales to poor people; and if, avoiding bad debts, he is able by strict economy to accumulate anything, it can be but a trifle. A large retail trader is obliged to lay out much money in fitting up an adequate establishment; he must invest a still greater sum in stock; and he must have a further floating capital to meet the charges that fall due before his returns come in. Setting out, however, with means enough for these purposes, he is able to make numerous and comparatively large sales; and so to get greater and more numerous increments of profit. Similarly, to get returns in thousands, merchants and manufacturers must make their investments in tens of thousands.
In brief, the rate at which a man's wealth accumulates, is measured by the surplus of income over expenditure; and this, save in exceptionably favourable cases, is determined by the capital with which he begins business. 'Now applying the analogy, we may trace in the transactions of an organism, the same three ultimate elements. There is the expenditure required for the obtainment and digestion of food; there is the gross return in the shape of nutriment assimilated, or fit for assimilation; and there is the difieience between this gross return of nutriment and the nutriment that was used up in the labour of securing it — a difference which may be a profit or a loss. Clearly, however, a surplus implies that the force expended is less than the force latent in the assimilated food. Clearly, too, the increment of growth is limited to the amount of this surplus of income over expenditure; so that large growth implies both that the excess of nutrition over waste shall be relatively considerable, and that the waste and nutrition shall be on extensive scales.
And clearly, the ability of an organism to expend largel}^ and assimilate largely, so as to make a large surplus, presupposes a large physiological capital, in the shape of organic matter more or less complete in its structural arrangements.
Throughout the vegetal kingdom, the illustrations of this truth are not conspicuous and regular: the obvious reason being, that since plants are accumulators and in so small a degree expenders, the premises of the above argument are but very partially fulfilled. The food of plants (excepting Fungi and certain parasites) being in a great measure the same for all, and bathing all so that it can be absorbed without effort, their vital processes result almost entirely in profit. Once fairly rooted in a fit place, a plant may thus from the outset add its entire returns to capital; and may soon be able to carry on its processes on a large scale, though it does not at first do so. When, however, plants are expenders, namely, during their germination and first stages of growth, their degrees of growth are determined by their amounts of vital capital. It is because the young tree commences life with a ready- formed embryo and store of food sufficient to last for some time, that it is enabled to strike root and lift its head above the surroundino- herbaore. Throuorhout the animal kingdom, however, the necessity of this relation is everywhere obvious. The small carnivoT-e preying on small herbivores, can increase in size only by small increments: its organization unfitting it to digest larger creatures, even if it can kill them, it cannot profit by amounts of nutriment exceeding a narrow limit; and its possible increments of growth being small to set out with, and rapidly decreasing, must come to an end before any considerable size is attained. Manifestly the young lion, born of tolerable bulk, suckled until much bigger, and fed until half- grown, is enabled by the power and organization which he thus gets gratis, to catch and kill animals of size enough to give him the large supply of nutriment needed to meet his large expenditure, and yet leave a large sur^^lus for growth. Thus then is explained GTtOAVTir. 131 the abovcnamed contrast between the ox and the sheep. A calf and a lamb commence their physiological transactions on widely different scales; their first increments of growth are similarly contrasted in their amounts; and the two diminishing series of such increments, end at similarly-contrasted limits.
§ 49. Such are the several conditions by which the phenomena of growth are governed. Conspiring and conflicting in endless different ways and degrees, they in every case qualify more or less differently each other's effects. Hence it happens that we are obliged to state each generalization as true on the average, or to make the proviso — other things equal.
Understood, in this qualified form, our conclusions are these. 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: this is alike a truth established by experience, and an inference from the truth given in our forms of thought [First Principles^ § 67). Second, that the available supply of assimilable matter being the same, and other conditions not dissimilar, the degree of growth varies according to the surplus of, nutrition over expenditure— a generalization which is illustrated in some of the broader contrasts between different divisions of organisms, and is a direct corollary from the persistence of force. 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. This proposition we found on the one hand exemplified by the unceasing growth of organisms that do not expend force; by the growth, slowly diminishing but never completely ceasing, of organisms that expend comparatively little force; and by the definitely limited growth of organisms that expend much force; and 132 THE INDrCTIONS OF BIOLOGY.
on the other hand, we found it to follow from a certain relative increase of expenditure that necessarily accompanies increase of bulk, and to be therefore an indirect corollary from the persistence of force. Fourth, that among organisms which are large expenders of force, the size ultimately attained is, other things equal, determined by the initial size: in proof of which conclusion we have abundant fects, as well as the a priori necessity that the sum-totals of analogous diminibhing series, must depend upon the amounts of their initial terms. Fifth, that where the likeness of other circumstances permits a comparison, the possible extent of growth depends on the degree of organization: an inference testified to by the larger forms among the various divisions and sub-divisions of organisms; and inferable a priori from the conditions of existence.
CHAPTEE II.
DEVELOPMENT.'* § 50. Certain general aspects of Development may be studied apart from any examination of internal structures. These fundamental contrasts between the modes of arrangement of parts, originating, as they do, the leading external distinctions among" the various forms of organization, will be best dealt with at the outset. If all organisms have arisen by Evolution, it is of course not to be expected that such several modes of development can be absolutely demarcated: we may be sure of finding them united by transitional modes. But premising that a classification of modes can but approximately represent the facts, we shall find our general conceptions of Development aided by one.
Development is primarily central. All organic forms of which the entire history is known, set out with a symmetrical arrangement of parts round a centre. In organisms of the lowest grade, no other mode of arrangement is ever definitely established; and in the highest organisms, central development, though subordinate to another mode of development, continues to be habitually shown in the changes of * In ordinary speech, Development is often used as synonymous with Growth. It hence seems needful to say, that Development as here and hereafter used, means increase of structure, and not increase of bulk. It may be added, that the word Evolution, comprehending Growth as well as Development, is to be reserved for occasions when both are implied.
minute structure. Let us glance at these propositions in the concrete. Leaving out those Rhizopods which are wholly structureless, every plant and animal in its earliest stage, consists of a spherical sac, full of liquid containing organic matter, in which is suspended a nucleated cell, more or less distinct from the rest; and the first changes that occur in the germ thus constituted, are changes that take place round centres produced by division of the original centre. From this type of structure, the simplest organisms do not depart; or depart in no definite or conspicuous ways. Among plants, the JJredo and the several tribes of Protococci permanently maintain such a central distribution; while among animals, it is permanently maintained by creatures like the Gregarina^ and in a different manner by the Amceha, ActinojohrySy and their allies. In larger organisms, made up chiefly of units that are analogous in structure to these simplest organisms, the formation of units ever continues to take place round points or nuclei; though the arrangement of these units into groups and wholes may proceed after another method.
Central development may be distinguished into luiicentral and multicentral; according as the product of the original germ, develops symmetrically round one centre, or develops without subordination to one centre — develops, that is, in subordination to many centres. Unicentral development, as displayed not in the formation of single cells but in the formation of aggregates, is not common. The animal kingdom shows it only in the small group named ThalassicoUce: inert, spherical masses of jelly, with scarcely any organization, which are found floating in southern seas. It is feebly represented in the vegetal kingdom by the Volvox ijlohator. On the other hand, multicentral development, or development round insubordinate centres, is variously exemplified in both divisions of the organic world. It is exemplified in two distinct ways, according as the insubordination among the centres of development is partial or total.
DEVELOrME.XT. 135 We njay most conveniently consider it under tlie heads lience arising.
Total insubordination among the centres of development, is shown where the units or cells, as fast as they are severally formed, part company and lead independent lives. This, in the vegetal kingdom, habitually occurs among the ProtopJujta; and in tlie animal kingdom, among the Protozoa. Partial insubordination is seen in those somewhat advanced organisms, that consist of units which, though they have not separated, have so little mutual dependence that the aggregate they form is irregular. Among plants, the Thallogens very generally exemplify this mode of development. Lichens, spreading with flat or corrugated edges in this or that direction, as the conditions de'termine, have no manifest co-ordination of parts. In the Algce, the Nostocs similarly show us an unsymmetrical structure. Of Fungi, the sessile and creeping kinds display no further dependence of one part on another, than is implied by their cohesion. And even in such better-organized plants as the Marchantia, the general arrangement shows no reference to a directive centre. Among animals, many of the Sponges may be cited as being thus devoid of that co-ordination implied by symmetry: the Amaeba-like units composing them, though they have some subordination to local centres, have no subordination to a general centre. To distinguish that kind of development in which the whole product of a germ coheres in one mass, from that kind of development in which it does not. Professor Huxley has introduced the words '^