the cambium-layer, represented by the end of the glove- finger; and supposing growth to continue in those parts of the cambium -layer that are nearer to the supply of nutri- ment, their further longitudinal extension will lead to the formation of a cavity at the extremity of the shoot, like that which results in a glove-finger when the finger is partially withdrawn and the glove sticks to its end. Whence it seems, with bract, but much smaller than the other; corolla large but cleft along the top; six stamens with anthers, pistil, and seed-vessel. 3rd flower, large; six-cleft calyx, cleft corolla, Avith six stamens, pistil, and seed-vessel, with a second pistil half unfolded at its apex. 4th flower, large; divided along the top, six stamens. 5th flower, large; corolla divided into three parts, six stamens. 6th flower, large; corolla cleft, calyx six -cleft, the rest of the flower normal. 7th, and all suc- ceeding flowers, normal.
228 THE INDUCTIONS OF BIOLOGY.
both that this introversion of the cambium-layer may be considered as due to failing nutrition, and that the ovules growing from its introverted surface (which would have been its outer surface but for the defective nutrition) are extremely aborted homologues of external appendages — either leaves or lateral axes: the essential organs of fructification thus arising where the defective nutrition has reached its extreme.* To all which let us not forget to add, that the sperm- cells and germ -cells are formed at the very ends of the organs of fruc- tification.
Those kinds of animals which multiply by heterogenesis, present us with a parallel relation between the recurrence- of gamogenesis and the recurrence of conditions unfavourable to growth — at least, this is shown where experiments have thrown light on the connexion of cause and effect; namely, among the Aphides. These creatures, hatched from eggs in the spring, multiply by agamogenesis throughout the summer. When the weather becomes cold, and plants no longer afibrd abundant sap, perfect males and females are produced; and from gamogenesis there result fertilized ova. But now observe that beyond this evidence, we have much more conclusive evidence. For it has been shown, both that the rapidity of the agamogenesis is proportionate to the warmth and nutrition, and that if the temperature and * It appears that botanists do not agree respecting the homologies of the o'v ules: some thinking that they are rudimentary foliar organs, and others that they are rudimentary axial organs. Possibly the dispute will prove a bootless one; since there seems evidence that ovules may be transformed into either one or the other. Mr Salter's paper, lately referred to, shows that they may graduate into stamens, which are foliar organs; and the case of the Foxglove, which I have described above, shows that they may develop into flower-buds, which are axial organs. I would venture to suggest, that the conflicting evidence can be reconciled, only by regarding ovules as the homologues of lateral append- ages; and considering a lateral appendage as composed of a leaf, plus a rudiment- ary axis, either of which may abort. This is the view which seems countenanced by development; since, in its first stage, a lateral bud, whence a lateral append- age arises, shows no division into rudimentary leaf and rudimentary axis; and it is to the lateral bud in this first stage, that the seed-bud or ovule is homo- logous.
GENESIS. 229 GENESIS. 229 supply of food bo ariidcially iniiiiitaincd, tlie agamogenesis coiitiiiiiGs through the winter. Nay more — it not only, under these conditions, continues through one winter, but it has been known to continue for four successive years: some forty or fifty sexless generations being thus produced. And those who have investigated the matter, see no reason to doubt the indefinite continuance of this agamogenetic mul- tiplication, so long as the external requirements are duly met. Evidence of another kind, which points very distinctly to the same conclusion, is furnished by the hetero- genesis of the Daplmia — a small crustacean commonly known as the Water-flea, which inhabits ponds and ditches. From the nature of its habitat, this little creature is exposed to very variable conditions. Besides being frozen up in winter, the small bodies of water in which it lives, are often unduly heated by the summer sun, or dried up by continued drought. The circumstances favourable to the DapJmid's life and growth, being thus liable to interruptions which, in our cli- mate, have a regular irregularity of recurrence; w^e may, in conformity with the hypothesis, expect to find both that the gamogenesis recurs along with evidence of declining nutri- tion, and that its recurrence is very -variable. This we do find. From Mr Lubbock's paper on the Daphnia in the " Philosophical Transactions " for 1857, and from further information which he -has been good enough to furnish me, the following general facts are deducible: — First, that in each ovarium, along with the rudiments of agamic eggs, or eggs which, if developed, produce young b}^ true partheno- genesis, there usually, if not always, exists the rudiment of an ephippial egg; which, from sundry evidences, is inferred to be a sexual or gamic Q^g. Second, that according to cir- cumstances, either agamogenesis or gamogenesis takes place; but that if the agamic eggs develop, the rudimentary gamic e^g disappears, or becomes absorbed; and conversely, if the gamic e^^ develops, the agamic eggs disappear, or are ab- sorbed by it. Third, that the brood of agamic eggs contained 230 THE INDUCTIONS OF BIOLOGY.
in each ovarium, amounts, under favourable circumstances, to as manj^ as eight or nine; while of the gamic eggs, only one at a time is produced in each ovarium, and occasionally one of the ovaria produces none: whence it follows, that as the gamic egg is not more than twice the bulk of the agamic egg J the quantity of matter contained in an agamic brood, is four times, and occasionally even eight times, as great as that contained in a gamic brood. Thus the quantity of nutriment expended in gamogenesis during a given period (making allowance for that which goes to the formation of the ephippium), is far less than that expended in agamogenesis during a like period. Seeing, then, this constant preparation for either gamic or agamic genesis, in a creature liable to such irregular variations of nutrition; and seeing that the agamogenesis implies by its amount, a large excess of nutri- tion, while the gamogenesis implies by its amount, a small excess of nutrition; we can scarcely doubt that the one or the other mode of multiplication occurs, according as the external conditions are or are not favourable to nutrition.
Passing now to animals which multiply by homogenesis — animals in which the whole product of a fertilized germ ag- gregates round a single centre or axis, instead of round many centres or axes; we see, as before, that so long as the con- ditions allow rapid increase in the mass of this germ-product, the formation of new individuals by gamogenesis does not take place. Speaking generally, we find that only when growth is declining in relative rapidity, do perfect sperm- cells and germ-cells begin to appear; and that the fullest activity of the reproductive function, arises as growth ceases — speaking generally, we must sa)^ because, though this relation is tolerably definite in the highest orders of animals which multiply by gamogenesis, it is less definite in the lower orders. This admission does not militate against the hypo- thesis, as it seems to do; for the indefiniteness of the relation occurs where the limit of growth is comparatively indefinite. We saw (§ 46) that among active, hot-blooded creatures.
r.ENESTS. 231 r.ENESTS. 231 sucli US mammals and birds, the inevitable balancing of assimilation by expenditure, establishes, for each species, an almost uniform adult size; and among creatures of these kinds, (birds especially, in which this restrictive effect of expenditure is most conspicuous), the connexion between cessation of growth and commencement of reproduction, is distinct. But we also saw (§ 46) that where, as in the Cro- codile and the Pike, the conditions and habits of life are such, that expenditure does not overtake assimilation as the size increases, there is no precise limit of growth; and in creatures thus circumstanced, we may naturally look for a compara- tively indeterminate relation between declining growth and commencing reproduction.* There is, indeed, among fishes, at least one case which appears very anomalous. The male parr, or young of the male salmon, a fish of four or five inches in length, is said to produce milt. Having, at this early stage of its growth, not one hundredth of the weight of a full-grown salmon, how does its production of milt consist with the alleged general law? The answer must be in a great measure hypothetical. If the salmon is (as it appears in its young state) a species of fresh -water trout, that has contracted the habit of annually migrating to the sea, where it finds a food on which it thrives — if the original size of this species was not much greater than that of the parr (which is nearly as large as some varieties of lake-trout and river- trout) — and if the limit of growth in the trout tribe is very indefinite, as we know it to be; then we may reasonably infer, that the parr has nearly the adult form and size of this species of. trout, before it acquired its migratory habit; and that this production of milt, is, * I owe to Mr Lubbock an important confirmation of this view. After stat- ing his belief, that between Crustaceans and Insects, there exists a physiological relation analogous to that which exists between water-vertebrata and land-verte- brata; he pointed out to me, that while among Insects, there is a definite limit of growth, and an accompanying definite commencement of reproduction, among Crustaceans, where growth has no definite limit, there is no definite relation between the commencement of reproduction and the decrease or arrest of growth.
232 THE INDUCTIONS OF BIOLOGY.
in such case, a concomitant of the incipient decline of growth naturally arising in the species, when living under the conditions of its remote ancestors. If this be admitted, the immense subsequent growth of the parr into the salmon, must be regarded as due to a suddenly -increased facility in obtaining food — a facility which removes to a great distance the limit at which assimilation is balanced by expenditure; and which has the effect, analogous to that produced in plants, of arresting the incipient reproductive process, and causing a resumption of growth. A confirmation of this view may be drawn from the fact, that when the parr, after its first migration to the sea, returns to fresh water, having increased in a few months from a couple of ounces to five or six pounds, it no longer shows any fitness for propagation: the grilse, or immature salmon, does not produce milt or spawn. But without citing further illustrations, or attempting to meet further difficulties, it has, I think, been made sufficiently clear, that some such connexion as that al- leged, exists. Traversed, as is this relation between commence- ment of sexual reproduction and declining rate of growth, by various other relations, it is quite as manifest as we can expect it to be.
The general law to which both homogenesis and hetero- genesis conform, thus appears to be, that the products of a fertilized germ go on accumulating by simple growth^ so long as the forces whence growth results are greatly in excess of the antagonist forces; but that when diminution of the one set of forces, or increase of the other, causes a considerable decline in this excess, and an approach towards equilibrium, fertilized germs are again produced. Whether the germ- product be organized round one axis, or round the many axes that arise by agamogenesis — whether the development be continuous or discontinuous; matters not. Whether, as in concrete organisms like the higher animals, this approach to equilibrium results from that disproportionate increase of expenditure entailed by increase of size; or whether, as in GENESIS. 23'] partially and wholly discrete organisms, like most plants and many inferior animals, this approach to equilibrium results from absolute or relative decline of nutrition; matters not. In any case, the recurrence of gamogenesis is associated with a more or less marked decrease in the excess of tissue-pro- ducing power. We cannot say, indeed, that a de- crease in this excess always results in gamogenesis; for we have evidence to the contrary, in the fact that some organ- isms multiply for an indefinite period by agamogenesis only. Thus, the weeping willow, which has been propagated through- out Europe, does not seed in Europe; and yet, as the weep- ing willow, by its large size and the multiplication of generation upon generation of lateral axes, presents the same causes of local innutrition as other trees, we cannot ascribe the absence of sexual axes to the continued predominance of nutrition. Among animals, too, the anomalous case of the Tineidce, a group of moths in which parthenogenetic mul- tiplication goes on for generation after generation, shows us that gamogenesis does not necessarily result from an approxi- mate balance of assimilation by expenditure. What we must say, is, that an approach towards equilibrium between the forces which cause growth and the forces which oppose growth, is the chief condition to the recurrence of gamo- genesis; but that there are other unknown conditions, in the absence of which this approach to equilibrium is not followed by gamogenesis.
§ 79. The above induction is an approximate answer to the question — When does gamogenesis recur? but not to the question which was propounded — Why does gamogenesis re- cur? — Why cannot multiplication be carried on in all cases, as it is in many cases, by agamogenesis? As already said, biologic science is not yet advanced enough to reply. Mean- while, the evidence above brought together, suggests a cer- tain hypothetical answer, which it may be well to set down.
Seeing as we do, on the one hand, that gamogenesis recurs 234 THE INDUCTIONS OF BIOLOGY.
only in individuals that are approaching towards a state of organic equilibrium; and seeing, on the other hand, as we do, that the sperm-cells and germ-cells thrown off by such individuals, are cells in which developmental changes have ended in quiescence, but in which, after their union, there arises a process of active cell- formation; we may suspect that the approach towards a state of general equilibrium in such gamogenetic individuals, is accompanied by an approach towards molecular equilibrium in them; and that the need for this union of sperm-cell and germ-cell, is the need for overthrowing this equilibrium, and re-establishing active mole- cular change in the detached germ — a result which is pro- bably effected by mixing the slightly different physiological units of slightly different individuals. The several argu- ments that may be brought in support of this view, cannot be satisfactorily set forth until after the topics of Heredity and Yariation have been dealt with. Leaving it for the pre- sent, I propose hereafter to reconsider this question, in con- nexion with sundry others that are raised by the phenomena of Genesis.
Before ending the chapter, however, it may be well to note the relations between these different modes of multiplication, and the conditions of existence under which they are respect- ively habitual. While the explanation of the teleologist is untrue, it is often an obverse to the truth; for though, on the hypothesis of Evolution, it is clear that things are not arranged thus or thus for the securing of special ends, it is also clear, that arrangements which do secure these special ends, tend continually to establish themselves — ai*e establish- ed by their fulfilment of these ends. Besides insuring a structural fitness between each kind of organism and its cir- cumstances, the working of " natural selection " also insures a fitness between the mode and rate of nudtiplication of each kind of organism and its circumstances. We may, therefore, without any teleological implication, consider the fitness of GENESIS. 235 homogencsls and hctcrogoncsis to the needs of the different classes of organisms which exhibit them.
One of the facts to be observed, is, that hcterogenesis pre- vails among organisms of which the food, though abundant compared with their expenditure, is dispersed in such a way that it cannot be appropriated in a wholesale manner. Pro- tophyta, subsisting on diffused gases and decaying organic matter in a state of minute subdivision; and Protozoa, to which food comes in the shape of extremely small floating particles; are enabled by their rapid agamogenetic multipli- cation, to obtain materials for growth, better than they would do did they not thus continually divide and disperse in pur- suit of it. The higher plants, having for nutriment the car- bonic acid of the air and certain mineral components of the soil, show us modes of multiplication adapted to the fullest utilization of these substances. A herb, with but little power of forming the woody-fibre requisite to make a stem that can support wide-spreading branches, after producing a few sex- less axes, produces sexual ones; and maintains its race better by the consequent early dispersion of seeds, than by a further production of sexless axes. But a tree, able to lift its suc- cessive generations of sexless axes high into the air, where each axis gets carbonic acid and light almost as freely as if it grew by itself, may with advantage go on budding-out sex- less axes year after year; since it thereby increases its sub- sequent power of budding- out sexual axes. Meanwhile, it may advantageously transform into seed-bearers, those axes which, in consequence of their less direct access to materials absorbed by the roots, are failing in their nutrition; for in doing this, it is throwing- off from a point at which sus- tenance is deficient, a migrating group of germs that may find sustenance elsewhere. The heterogenesis displayed by animals of the Coelenterate type, has evidently a like utility. A polype, feeding on minute annelids and crustaceans, which, flitting through the water, come in contact with its tentacles; 236 " THE INDUCTIONS OF BIOLOGY.
and limited to that quantity of prey which chance brings within its grasp; buds out young polypes which, either as a colony or as dispersed individuals, spread their tentacles through a larger space of water than the parent alone can; and by producing them, the parent better insures the continu- ance of its species, than it would do if it went on slowly grow- ing until its nutrition was nearly balanced by its waste, and then multiplied by gamogenesis. Similarly with the Aphis. Living on sap sucked through its proboscis from tender shoots and leaves, and able thus to take in but a very small quan- tity in a given time, this creature's race is more likely to be preserved by a rapid asexual propagation of small indi- viduals, which disperse themselves over a wide but nowhere rich area of nutrition, than it would be did the individual growth continue so as to produce large individuals multiply- ing sexually. While at the same time we see, that when autumnal cold and diminishing supply of sap, put a check to growth, the recurrence of gamogenesis, and production of fertilized ova that remain dormant through the winter, is more favourable to the preservation of the race, than would be a further continuance of agamogenesis. On the other hand, it is obvious that among the higher animals, living on food which, though dispersed, is more or less aggregated into large masses, this alternation of gamic and agamic reproduction ceases to be useful. The development of the germ-product into a single organism of considerable bulk, is in many cases a condition without which these large masses of nutriment could not be appropriated; and here the formation of many individuals instead of one, would be fatal. But we still see the beneficial results of the general law — the postponement of gamogenesis until the rate of growth begins to decline. For so long as the rate of growth continues rapid, it is a proof that the organism gets food with great facility — that expenditure is not such as seriously to check accumulation; and that the size reached is as yet not disad- vantageous— or rather, indeed, that it is advantageous. But GENESIS. 237 GENESIS. 237 when the rate of growth is much decreased by the compara- tively rapid increase of expenditure — when the excess of assimilative power is diminishing in such a way as to indi- cate its approaching disappearance; it becomes needful for the maintenance of the species, that this excess shall be turned to the production of new individuals; since, did growth continue until this excess disappeared through the complete balancing of assimilation and expenditure, the pro- duction of new individuals would be either impossible or fatal to the parent. And it is clear that " natural selection " will continually tend to determine the period at which gamo- genesis commences, in such a way as most favours the main- tenance of the race.
Here, too, may fitly be pointed out the fact, that, by ** natural selection," there will in every case be produced, the most advantageous proportion of males and females. If the conditions of life are such as to render a greater or less in- equality of the sexes beneficial to the species, in respect either of the number of the offspring, or the character of the offspring; then, those varieties of the species which, from any cause, approach more than other varieties towards this beneficial degree of inequality, will be apt to supplant other varieties. And conversely, where equality in the number of males and females is beneficial, the equilibrium will be main- tained by the dying out of such varieties as produce offspring among which the sexes are not balanced.
CHAPTER VIII.
HEREDITY.
§ 80. Already, in the last two chapters, the law of heredi- tary transmission has been tacitly assumed; as, indeed, it unavoidably is in all such discussions. Understood in its entirety, the law is, that each plant or animal produces others of like kind with itself: the likeness of kind consist- ing, not so much in the repetition of individual traits, as in the assumption of the same general structure. This truth has been rendered so familiar by daily illustration, as almost to have lost its significance. That wheat produces wheat — that existing oxen have descended from ancestral oxen — that every unfolding organism eventually takes the form of the class, order, genus, and species from which it sprang; is a fact which, by force of repetition, has acquired in our minds almost the aspect of a necessity. It is in this, however, that Heredity is principally displayed: the phenomena com- monly referred to it, being quite subordinate manifestations. And, as thus understood, Heredity is universal. The various instances of heterogenesis lately contemplated, seem, indeed, to be at variance with this assertion. But they are not really so. Though the recurrence of like forms, is, in these in- stances, not direct but cyclical, still, the like forms do recur; and when taken together, the group of forms produced during one of the cycles, is as much like the groups produced in pre- ceding cycles, as the single individual arising by homo- genesis, is like ancestral individuals.
HEREDITY. 239 While, however, the general truth that organisms of a given type uniformly descend from organisms of the same type, is so well established by infinite illustrations, as to have assumed the character o£ an axiom; it is not universally admitted that non-typical peculiarities are inherited. While the botanist would be so incredulous if told that a plant of one class had produced a plant of another class, or that from seeds belonging to one order individuals belonging to another order had grown, that he would deem it needless to examine the evidence; and while the zoologist would treat with con- tempt the assertion, that from the egg of a fish a reptile had arisen, or that an implacental mammal had borne a pla- cental mammal, or that an unguiculate quadruped had sprung from an ungulate quadruped, or even that from individuals of one species offspring of an allied species had proceeded; yet there are botanists and zoologists who do not consider it certain, that the minor specialities of organization are trans- mitted from one generation to another. Some naturalists seem to entertain a vague belief, that the law of Heredity applies only to main characters of structure, and not to de- tails; or, at any rate, that though it applies to such details as constitute differences of species, it does not apply to smaller details. The circumstance that the tendency to re- petition, is in a slight degree qualified by the tendency to variation (which, as we shall hereafter see, is but an indirect result of the tendency to repetition), leads some to doubt whether Heredity is unlimited. A careful weighing of the evidence, however, and a due allowance for the influences by which the minuter manifestations of Heredity are obscured, will remove the grounds for this scepticism.