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 240 THE INDUCTIONS OF BIOLOGY.
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 pecu- liarities 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 per- manent establishment of races that have diverged widely from a common stock. Among fruits and flowers, the multi- plication of kinds, and the continuance of each kind with certainty by agamogenesis, and to some extent by gamo- genesis, 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 physiolo- gical test, we must consider as all of one species, show us in a marked manner the hereditary transmission of small difier- ences — 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 gener- ation. Or grant only that there is evidence of their deriva- tion from several stocks, and we still have, between races de- scended from a common stock, distinctions which prove the inheritance of minor peculiarities. Besides seeing that HEREDITY. 241 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 broad- faced 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 differ- ent from that common in Britain: near akin though the Germans are to the British. The average Italian face con- tinues 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 per- petuation 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 be- lief derives from evidence of this kind, it has also the support of still more special evidence. Numerous illustrations of He- redity 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 peculiar- ities 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 testi- mony. Excluding those inductions that have been so fully 16 242 THE INDUCTIONS OF BIOLOGY.
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 genera- tion 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 in- herited 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 in- ferior 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 agri- culturist 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 exist- ence." 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 dis- tinctions.
Of special instances, there are many besides that of the oft- en-cited Otter-breed of sheep, descended from a single short- logged 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 con- tributed 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, be- queathed this malformation, along some lines of her descend- ants, 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 grand- father 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 mon- strosity 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 * 244 THE INDUCTIONS OF BIOLOGY.
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 am- aurosis 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 trans- mitted from parent to child. There are deaf-mutes whose imperfections have been derived from ancestors; and mal- formations 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, sebace- ous tumours, plica polonica, dipsomania, somnambulism, cata- lepsy, 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 pecu- liarities 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, here- ditary; hut that a peculiarity of vision confined to one eye, is frequently trans- mitted— re -appearing in the same eye in offspring.
_J i HERKDITY. 245 for several reasons, comparatively difficult. Changes pro- duced 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 un- less 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 special- ities 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 in- duced 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 droop- ing character. Now these " changes of habit" are clearly de- termined by functional changes. Occurring, as they do, in many individuals that have undergone the same transportation.
246 THE INDUCTIONS OF BIOLOGY.
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-ap- pear 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 un- like 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 pro- HEREDITY. 247 portion 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 develop- ment 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 diifer- ent classes, which inhabit the caves of Styria and of Kentucky, are blind. In some of the crabs the footstalk of the eye re- mains, 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 ac- quired 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 under- gone 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 Ameri- canized. This cannot be ascribed to intermarriage with Americans; since the feeling with which Irish are regard- ed by Americans, prevents any considerable amount of inter- marriage. 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 numer- ous, 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 so- cial 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 re- sidence 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 con- nexion, 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 large- ness of the feet, we have strong evidence in the customs of the Chinese. The torturing practice of artificiall}' arresting the 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 no- toriously 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.
Some of the best illustrations of functional hereditj'', are furnished by the mental characteristics of human races. Cer- tain powers which mankind have gained in the course of civil- ization, cannot, I think, be accounted for, without admitting the inheritance of acquired modifications. The musical faculty is one of these. To say that " natural selection" has developed it, by preserving the most musically endowed, seems an in- adequate explanation. Even now that the development and prevalence of the facultj^ have made music an occupation by which the most musical can get sustenance and bring up families; it is very questionable whether, taking the musical life as a whole, it has any advantage over others in the struggle for existence and multiplication. Still more if we look back to those early stages through which the faculty must have passed, before definite perception of melody was arrived at, we fail to see how those possessing the rudimental faculty in a somewhat greater degree than the rest, would thereby be enabled the better to maintain themselves and their children. If so, there is no explanation but that the habitual association of certain cadences of human speech with certain emotions, has slowly established in the race an organized and inherited connexion between such cadences and such emotions; that the 250 THE INDUCTIONS OF BIOLOGY.
combination of sucli cadences, more or less idealized, which constitutes melody, has all along had a meaning in the average mind, only because of the meaning which cadences had acquired in the average mind; and that by the continual hearing and practice of melody, there has been gained and transmitted an increasing musical sensibility. Confirmation of this view may be drawn from individual cases. Grant that among a people endowed with musical faculty to a certain degree, spontaneous variation will occasionally produce men possessing it in a higher degree; it cannot be granted that spontaneous variation accounts for the frequent production, by such highly- endowed men, of men still more highly endowed. On the average, the offspring of marriage with others not similarly endowed, will be less distinguished rather than more distin- guished. The most that can be expected is, that this unusual amount of faculty shall re-appear in the next generation undi- minished. How then shall we explain cases like those of Bach, Mozart, and Beethoven, who were all sons of men having un- usual musical powers, but who greatly excelled their fathers in their musical powers? What shall we say to the facts, that Haydn was the son of the organist, that Hummel was born to a music master, and that Weber's father was a dis- tinguished violinist? The occurrence of so many cases in one nation, within a short period of time, cannot rationally be ascribed to the coincidence of " spontaneous variations." It can be ascribed to nothing but inherited developments of structure, caused by augmentations of function.
But the clearest proof that structural alterations caused by alterations of function, are inherited, occurs when the alter- ations are morbid. " Certain modes of living engender gout; " and gout is transmissible. It is well known that in persons pre- viously healthy, consumption may be produced by unfavourable conditions of life — by bad and insufficient food; by foul, damp, unventilated habitations; and even by long- continued anxiety. It is still more notorious that the consumptive diathesis is conveyed from parent to child. Unless, then, a distinction HKREDITY. 251 be assumed between constitutional consumption and con- sumption induced by unwholesome conditions — unless it bo asserted that consumption of unknown origin is transmiss- ible, while functionally-produced consumption is not; it must be admitted that those changes of structure from which the consumptive diathesis results, may be caused in parents by changes of function, and may be inherited by their chil- dren. Most striking of all, however, is the fact lately brought to light, that functional disorders artificially estab- lished, may be. conveyed to offspring. Some few years since M. Brown-Sequard, in the course of inquiries into the nature and causes of epilepsy, hit on a method by which epilepsy could be orififinated. Guinea- dij^s were the creatures on which, chiefly, he experimented; and eventually, he disco- vered the remarkable fact, that the young of these epileptic guinea-pigs were epileptic: the functionally-established epilepsy in the parents, became constitutional epilepsy in the offspring. Here we have an instance which, standing even alone, decides the question. We have a special form of nervous action, not caused by any natural variation of structure that had arisen spontaneously in the organism, but one caused by a certain incidence of external forces. We have this