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 sustenance 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 continuance of its species, than it would do if it went on slowly growing 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 quantity in a given time, this creature's race is more likely to be preserved by a rapid asexual propagation of small individuals, 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 multiplying 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 disadvantageous— or rather, indeed, that it is advantageous. But when the rate of growth is much decreased by the comparatively rapid increase of expenditure — when the excess of assimilative power is diminishing in such a way as to indicate 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 production 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 gamogenesis commences, in such a way as most favours the maintenance 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 inequality 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 maintained by the dying out of such varieties as produce offspring among which the sexes are not balanced.
CHAPTER VIII.
§ 80. Already, in the last two chapters, the law of hereditary 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 consisting, 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 commonly 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 instances, 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 preceding cycles, as the single individual arising by homogenesis, is like ancestral individuals.
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 contempt the assertion, that from the egg of a fish a reptile had arisen, or that an implacental mammal had borne a placental 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 transmitted 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 details; 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 repetition, 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.
First in order of importance, comes the fact, that not only are there uniformly transmitted from an organism to its ofispring, those traits of structure which distinguish the class, order, genus, and species; but also those which distinguish the variety. We have numerous cases, among both plants and animals, where, by natural or artificial conditions, there have been produced divergent modifications of the same species; and abundant proof exists that the members of any one sub-species, habitually transmit their distinctive peculiarities to their descendants. Agriculturists and gardeners can furnish unquestionable illustrations. Several varieties of wheat are known; of which each reproduces itself. Since its introduction into England, there have been formed from the potato, a number of sub-species: some of them differing greatly in their forms, sizes, qualities, and periods of ripening. Of peas, also, the like may be said. And the case of the cabbage-tribe, is often cited as showing the permanent establishment of races that have diverged widely from a common stock. Among fruits and flowers, the multiplication of kinds, and the continuance of each kind with certainty by agamogenesis, and to some extent by gamogenesis, might be exemplified without end. From all sides evidence may be gathered showing a like persistence of varieties in each species of animal. We have our distinct breeds of sheep, our distinct breeds of cattle, our distinct breeds of horses: each breed maintaining its characteristics.
The several sorts of dogs, which, if we accept the physiological test, we must consider as all of one species, show us in a marked manner the hereditary transmission of small difierences — each sort, when kept pure, reproducing itself not only in size, form, colour, and quality of hair, but also in disposition and speciality of intelligence. Rabbits, too, have their permanently- established races. And in the Isle of Man, we have a tail-less kind of cat. Even in the absence of other evidence, that which ethnology furnishes would suffice. Grant them to be derived from one stock, and the varieties of man yield proof upon proof that non-specific traits of structure are bequeathed from generation to generation. Or grant only that there is evidence of their derivation from several stocks, and we still have, between races descended from a common stock, distinctions which prove the inheritance of minor peculiarities. Besides seeing that negroes continue to produce negroes, copper-coloured men to produce men of a copper colour, and the fair-skinned races to perpetuate their fair skins — besides seeing that the broadfaced and flat-nosed Calrauck begets children with broad faces and flat noses, while the Jew bequeaths to his offspring the features which have so long characterized Jews; we see that those small unlikenesses which distinguish more nearly-allied varieties of men, are maintained from generation to generation. In Germany, the ordinary shape of skull is appreciably different from that common in Britain: near akin though the Germans are to the British. The average Italian face continues to be unlike the faces of northern nations. The French character is now, as it was centuries ago, contrasted in sundry respects with the characters of neighbouring peoples. Nay, even between races so closely allied as the Scotch Celts, the Welch Celts, and the Irish Celts, appreciable differences of form and nature have become established.
That sub-species and sub-sub-species, thus exemplify that same general law of inheritance which shows itself in the perpetuation of ordinal, generic, and specific peculiarities; is strong reason for the belief that this general law is unlimited in its application. In addition to the warrant which this belief derives from evidence of this kind, it has also the support of still more special evidence. Numerous illustrations of Heredity are yielded by experiment, and by direct observation of successive generations. They are divisible into two classes. In the one class come cases where congenital peculiarities, not traceable to any obvious causes, are bequeathed to de-'i scendants. In the other class come cases where the peculiarities thus bequeathed are not congenital, but have resulted from changes of functions during the lives of the individuals bequeathing them. We will consider first the cases that come in the first class.
§ 81. Note at the outset the character of the chief testimony. Excluding those inductions that have been so fully verified as to rank with exact science, there are no inductions so trustworthy as those which have undergone the mercantile test. "When we have thousands of men whose profit or loss depends on the truth of the inferences they draw from simple and perpetually- repeated observations; and when we find that the inference arrived at, and handed down from generation to generation of these deeply-interested observers, has become an unshakable conviction; we may accept it without hesitation. In breeders of animals we have such a class, led by such experiences, and entertaining such a conviction — the conviction that minor peculiarities of organization are inherited as well as major peculiarities. Hence the immense prices given for successful racers, bulls of superior forms, sheep that have certain desired peculiarities. Hence the careful record of pedigrees of high-bred horses and sporting dogs. Hence the care taken to avoid intermixture with inferior stocks. Citing the highest authorities respecting the effects of breeding from animals having certain superiorities, with the view of propagating those superiorities, Mr Darwin writes: — *' Youatt, who was probably better acquainted with the works of agriculturists than almost any other individual, and who was himself a very good judge of an animal, speaks of the principle of selection as ' that which enables the agriculturist not only to modify the character of his flock, but to change it altogether. It is the magician^s wand, by means of which he may summon into life whatever form and mould he pleases.' ^^ Lord Somerville, speaking of what breeders have done for sheep, says: — '' It would seem that they had chalked upon a wall a form perfect in itself and then given it existence." That most skilful breeder, Sir John Sebright, used to say, with respect to pigeons, that "he would produce any given feather in three years, but it would take him six years to obtain head and beak." In all which statements the tacit assertion is, that individual traits are bequeathed from generation to generation; and that when they are not brought into conflict with opposite traits, they may be HEREDITY. ^43 SO perpetuated and increased as to become permanent distinctions.
Of special instances, there are many besides that of the often-cited Otter-breed of sheep, descended from a single shortlogged lamb, and that of the six-fingered Gratio Kelleia, who transmitted his peculiarity in different degrees, to several of his children and to some of his grandchildren. In a paper contributed to the Edinburgh New Philompliical Journal for July 1863, Dr Struthers gives several cases of hereditary digital variations. Esther P —, who had six fingers on one hand, bequeathed this malformation, along some lines of her descendants, for two, three, and four generations. A — S — inherited an extra digit on each hand and each foot from his father; and C — Gf —, who also had six fingers and six toes, had an aunt and a grandmother similarly formed. A collection of evidence has been made by Mr Sedgwick, and published by him in the Medico- Chirurgical Review for April and for July 1863, in two articles on " The Influence of Sex in limiting Hereditary Transmission.'^ From these articles are selected the following cases and authorities: — ^Augustin Duforet, a pastry-cook of Douai, who had but two instead of three phalanges to all his fingers and toes, inherited this malformation from his grandfather and father, and had it in common with an uncle and numerous cousins. An account has been given by Dr Lepine, of a man with only three fingers on each hand and four toes on each foot, and whose grandfather and son exhibited the like anomaly. Bechet describes Yictoire Barre as a woman who, like her father and sister, had but one developed finger on each hand, and but two toes on each foot, and whose monstrosity re-appeared in two daughters. And there is a case where the absence of two distal phalanges on the hands w^as traced for two generations. The various recorded instances in which there has been transmission from one generation to another, of webbed-fingers, of webbed-toes, of hare-lip, of congenital luxation of the thigh, of absent patellae, of club-foot, &c., would occupy more space than can here be IG * spared. Defects in the organs of sense are also not unfrequently inherited. Four sisters, their mother, and grandmother, are described by Duval as similarly affected by cataract. Prosper Lucas details an example of hereditary amaurosis affecting the females of a family for three generations. Duval, Graffe, Dufon, and others testify to like cases coming under their observation.* Deafness, too, is occasionally transmitted from parent to child. There are deaf-mutes whose imperfections have been derived from ancestors; and malformations of the external ears have also been perpetuated in offspring. Of transmitted peculiarities of the skin and its appendages, many illustrations have been noted. One is that of a family remarkable for enormous black eyebrows; another that of a family in which every member had a lock of hair of a lighter colour than the rest on the top of the head; and there are also instances of congenital baldness being hereditary. Entire absence of teeth, absence of particular teeth, and anomalous arrangements of teeth, are recorded as traits that have descended to children. And we have evidence that soundness and unsoundness of teeth are transmissible.
The inheritance of such diseases as gout, consumption, and insanity, is univ^ersally admitted. Among the less-common diseases of which the descent from one generation to another has been observed, are, ichthyosis, leprosy, pityriasis, sebaceous tumours, plica polonica, dipsomania, somnambulism, catalepsy, epilepsy, asthma, apoplexy, elephantiasis. General nervousness displayed by parents, almost always re-appears in their children. Even a bias towards suicide appears to be sometimes hereditary.
§ 82. To prove the transmission of those structural peculiarities that have resulted from functional peculiarities, is, * "While this chapter is passing through the press, I learn from Mr White Cooper, that not only are near sight, long sight, dull sight, and squinting, hereditary; hut that a peculiarity of vision confined to one eye, is frequently transmitted— re -appearing in the same eye in offspring.
_J i HERKDITY. 245 for several reasons, comparatively difficult. Changes produced in the sizes of parts by changes in their amounts of action, are mostly unobtrusive. A muscle that has increased in bulk, is so obscured by natural or artificial clothing, that unless the alteration is extreme it passes without remark. Such nervous developments as are possible in the course of a single life, cannot be seen externally. Visceral modifications of a normal kind, are observable but obscurely, or not at all. And if the changes of structure worked in individuals by changes in their habits, are thus difficult to trace; still more difficult to trace must be the transmission of them — further hidden, as this is, by the influence of other individuals that are often otherwise modified by other habits. Moreover, such specialities of structure as are due to specialities of function, are usually entangled with specialities of structure that are, or may be, due to selection, natural or artificial. In the majority of cases, it is impossible to say that a structural peculiarity which seems to have arisen in offspring from a functional peculiarity in the parent, is wholly independent of some congenital peculiarity of structure in the parent, which induced this functional peculiarity. We are restricted to cases with w^hich natural or artificial selection can have had nothing to do; and such cases are difficult to find. Some, however, may here be noted.
A species of plant that has been transferred from one soil or climate to another, frequently undergoes what botanists call " a change of habit " — a change which, without affecting its specific characters, is yet conspicuous. In its new locality, the species is distinguished by leaves that are much larger, or much smaller, or differently shaped, or more fleshy; or instead of being, as before, comparatively smooth, it becomes hairy; or its stem becomes woody instead of being herbaceous; or its branches, no longer growing upwards, assume a drooping character. Now these " changes of habit" are clearly determined by functional changes. Occurring, as they do, in many individuals that have undergone the same transportation.
they cannot be classed as '' spontaneous variations." They are modifications of structure, consequent on modifications of function, that have been produced by modifications in the actions of external forces. And as these modifications re-appear in succeeding generations, we have, in them, examples of functionally-established variations that are hereditarily transmitted. Further evidence is supplied by what are called '' sports " in plants. These are of two kinds — the gamogenetic and the agamogenetic. The gamogenetic may be ascribed wholly to '' spontaneous variations;" or if they are partly due to the inheritance of structural changes that are produced by functional changes, this cannot be proved. But where the individuals displaying the variations arise by agamogenesis, the reverse is the case: spontaneous variation is out of the question; and the only possible interpretation is deviation of structure caused by deviation of function. A new axis which buds out from a parent-axis, assumes an unlike character — gives off lobed leaves in place of single leaves, or has an otherwise different mode of growth. This change of structure implies change in the developmental actions which produced the new bud — change, that is, in the actions going on in the parent shoot — functional change. And since the modified structure thus impressed on the new shoot by modified function, is transmitted by it to all the shoots it bears; we are obliged to regard the case as one of acquired modification that has become hereditary.
Evidence of analogous changes in animals, is difficult to disentangle. Only among domesticated animals, have we any opportunity of tracing the effects of altered habits; and here, in nearly all cases, artificial selection has obscured the results.
Still, there are some facts which seem to the point. Mr Darwin, while ascribing almost wholly to " natural selection '* the production of those modifications which eventuate in differences of species, nevertheless admits the effects of use and disuse. He says — " I find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the wliolo skeleton, than do the same bones in the wild duck; and I presume that this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parent. The great and inherited development of the udders in cows and goats in countries where they are habitually milked, in comparison with the state of these organs in other countries, is another instance of the effect of use. Not a single domestic animal can bo named which has not in some country drooping ears; and the view suggested by some authors, that the drooping is due to the disuse of the muscles of the ear, from the animals not being much alarmed by danger, seems probable." Again — " The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered up by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection." * * * '^ It is well known that several animals, belonging to the most diiferent classes, which inhabit the caves of Styria and of Kentucky, are blind. In some of the crabs the footstalk of the eye remains, though the eye is gone; the stand for the telescope is there, though the telescope with its glasses has been lost. As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse." The direct inheritance of an acquired peculiarity is sometimes observable. Mr Lewes gives a case. He " had a pnppy taken from its mother at six weeks old, who, although never taught, ' to beg' (an accomplishment his mother had been taught), spontaneously took to begging for everything he wanted when about seven or eight months old: he would beg for food, beg to be let out of the room, and one day was found opposite a rabbit hutch begging for rabbits." Instances are on record, too, of sporting dogs which spontaneously adopted in the field, certain modes of behaviour which their parents had learnt.
But the best examples of inherited modifications produced by modifications of function, occur in the human race. To no 248 ' THE INDUCTIONS OF BIOLOGY.
other cause cau be ascribed the rapid metamorphoses undergone by the British races when placed in new conditions. It is notorious that, in the United States, the descendants of the immigrant Irish lose their Celtic aspect, and become Americanized. This cannot be ascribed to intermarriage with Americans; since the feeling with which Irish are regarded by Americans, prevents any considerable amount of intermarriage. Equally marked is the case of the immigrant Germans, who, though they keep themselves very much apart, rapidly assume the prevailing type. To say that *' spontaneous variation " increased by natural selection, can have produced this effect, is going too far. Races so numerous, cannot have been supplanted in the course of two or three generations by varieties springing from them. Hence there is no escape from the conclusion, that physical and social conditions have here wrought modifications of function and structure, which offspring have inherited and increased. Similarly with special cases. In the Cyclopcedia of Practical Medicine^ Vol. II. p. 419, Dr Brown states that he "has in many instances observed in the case of individuals whose complexion and general appearance has been modified by residence in hot climates, that children born to them subsequently to such residence, have resembled them rather in their acquired than primary mien."
Some special modifications of organs caused by special changes in their functions, may also be noted. That large hands are inherited by men and women whose ancestors led laborious lives; and that men and women whose descent, for many generations, has been from those unused to manual labour, commonly have small hands; are established opinions. It seems ver}^ unlikely that in the absence of any such connexion, the size of the hand should thus have come to be generally regarded as some index of extraction. That there exists a like relation between habitual use of the feet and largeness of the feet, we have strong evidence in the customs of the Chinese. The torturing practice of artificiall}' arresting the HEHEUITY. 249 HEHEUITY. 249 growth of tlio feet, could never have become established among the ladies of China, had they not found abundant proof that a small foot was significant of superior rank — that is of a luxurious life — tliat is of a life without bodily labour. There is some evidence, too, that modifications of the eyes, caused by particular uses of the eyes, are inherited. Short sight appears to be uncommon in rural populations; but it is frequent among classes of people who use their eyes much for reading and writing; and in these classes, short sight is often congenital. Still more marked is this relation in Germany. There, the educated classes are notoriously studious; and judging from the numbers of young Germans who wear spectacles, there is reason to think that congenital myopia is very frequent among them.