124 THE INDUCTIONS OF BIOLOGY.
however, thougli the chief, is not the sole, varying relation be- tween 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 ex- pended 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 sub- stance, 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 in- crease only as the squares of the dimensions, it certainly in- creases 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 pro- duced during action, that the loss of it is then of no conse- quence: 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 GROWTH. 125 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 apprecia- ble 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 de- duction, 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 con- siderably. 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 organ- ization, have no definite limits of growth, is, however, an ex- ceptional 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 cold- blooded, creature; that is, it expends very little for the main- tenance 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 in- creasing as the cubes of its dimensions, which it has thus to overcome, is the inertia of its parts. The exceptional con- tinuance of growth observed in creatures so circumstanced^ is therefore perfectly explicable.
f GROWTH. 127 § 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 after- processes requisite for separating the nutritive atoms — an exertion which therefore varies according as the food is abund- ant or scarce, fixed or moving, according as it is mechani- cally easy or difficult to deal with when secured, and accord- ing 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 be- tween 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 con- spiring causes which affect growth are so numerous. The only contrast which seems fairly to the point, is the before- named 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 rep- tiles, 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 128 THE INDUCTIONS OF BIOLOGY.
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 de- termine 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 tur- nips, obtain these aliments witii the same small expenditure of force, and differ scarcely at all in their degrees of organiz- ation. Whence arises, then, their striking unlikeness of bulk?
We noted when studying the phenomena of growth in- ductively, that organisms of the larger and higher types, com- mence their separate existences, as masses of organic matter having tolerable magnitudes. Speaking generally, we saw that throughout each organic sub-kingdom, the acquire- ment 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: leav- ing him to start on the morrow in the same condition as GROWTH. 129 before. The trade of the huxter in ounces of tea and half- pounds 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, avoid- ing 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 manu- facturers 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 as- similated, 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.
9 130 THE INDUCTIONS OF BIOLOGY.
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 with- out 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 ex- ceeding 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 un- til 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 diminish- ing series of such increments, end at similarly-contrasted limits.
§ 49. Such are the several conditions by which the phe- nomena 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 de- pendent 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 ex- penditure— a generalization which is illustrated in some of the broader contrasts between different divisions of organ- isms, 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 rela- tive increase of expenditure that necessarily accompanies in- crease 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 at- tained 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 cir- cumstances 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 arrange- ment 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 approx- imately represent the facts, we shall find our general con- ceptions of Development aided by one.
Development is primarily central. All organic forms of which the entire history is known, set out with a symmetri- cal 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 de- velopment, 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.
134 THE INDUCTIONS OF BIOLOGY.
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 crea- tures 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 Vol- vox ijlohator. On the other hand, multicentral development, or development round insubordinate centres, is va- riously exemplified in both divisions of the organic world. It is exemplified in two distinct ways, according as the insubor- dination 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 Proto- pJujta; and in tlie animal kingdom, among the Proto- zoa. Partial insubordination is seen in those somewhat advanced organisms, that consist of units which, though they have not separated, have so little mutual depend- ence 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 '^ con- tinuous '' and " discontinuous;^^ and these seem the best fitted for the purpose. Multicentral development, then, is divisible into continuous and discontinuous.
From central development we pass insensibly to that higher kind of development for which axial seems the most appro- priate name. A tendency towards this is vaguely manifested almost everywhere. The great majority even of Protophyta and Protozoa have different longitudinal and transverse di- 136 THE INDUCTIONS OF BIOLOGY.
mensioiis — have an obscure if not a distinct axial structure. The originally cellular units out of which higher organisms are mainly built up, usually pass into shapes that are subordi- nated to lines rather than to points. And in the higher organ- isms, considered as wholes, an arrangement of parts in rela- tion to an axis is distinct and nearly universal. AVe see it in the superior orders of Thallogens; and in all the Acrogens, Endogens, and Exogens. With few exceptions the Coelente- rata clearly exhibit it; it is traceable, though less conspicu- ously, throughout the Mollusca; and the Annulosa and Vertehrata uniformly show it with perfect definiteness.
This kind of development, like the first kind, is of two orders. The whole germ-product may arrange itself round a single axis, or it may arrange itself round many axes; the structure may be uniaxial or multiaxial. Each division of the organic kingdom furnishes examples of both these or- ders. In such Fungi as exhibit axial development at all, we commonly see development round a single axis. Some of the Algce, as the common tangle, show us this arrange- ment. And of the higher plants, many Endogens and small Exogens are uniaxial. Of animals, the advanced are without exception in this category. There is no known ver- tebrate in which the whole of the germ-product is not subor- dinated to a single axis. In the more fully- organized Annu- losa, the like is almost universal; as it is also in the superior orders of Mollusca. Multiaxial development occurs in most of the plants we are familiar with — every branch of a shrub or tree being an independent axis. But while in the vegetal kingdom, multiaxial development prevails among the highest types; in the animal kingdom, it prevails only among the lowest types. It is extremely general, if not universal, among the Ccelenterata; it is characteristic of the Mollus- coida.; among Molluscs the compound Ascidians exhibit it; and it is seen, though under another form, in the inferior Annulosa, Development that is axial, like development that is central, DEVKLOPMEiXT. 107 may be eltlicr continuous or discontinuous: the parts having different axes may continue united, or they may separate. Instances of each alternative are supplied by both plants and animals. Continuous, multlaxial development, is that which plants usually display; and need not be illustrated further than by reference to every garden. As cases of it in animals may be named, all the compound Hydrozoa and Ac- tinozoa; and such molluscous forms as the BotryUidce. Of