special form of nervous action becoming confirmed by re- petition: the fits are more and more easily induced — there is established the epileptic hahiL That is to say, the connected nervous actions constituting a fit, produce in the nervous system such changes of struct are, that subsequent connected nervous actions of like kind, follow one another with increased readiness. And that this epileptic habit is inherited, proves conclusively that these structural modifications worked by functional modifications, are impressed on the whole organism in such way as to affect the reproductive centres, and cau^e them to unfold into organisms that exhibit like modifications. Evidence nearly allied to this, and scarcely less significant, is furnished by that transmission of general nervousness, no- ticed in the last section. Nervousness is especially common 252 THE INDUCTIONS OF BIOLOGY.
among classes of people who tax their brains much. Among these classes, we daily see this constitutional modification produced by excess of function, in men whose progenitors were not nervous; and the children of such men habitually inherit more or less of the modification.
§ 83. Two modified manifestations of Heredity remain to be noticed. The one is the re-appearance in offspring, of traits not borne by the parents, but borne by the grandparents or by remoter ancestors. The other is the limitation of Heredity by sex — the restriction of certain transmitted peculiarities to offsjjring of the same sex as the parent possessing these peculiarities.
Atavism, which is the name given to the recurrence of ancestral traits, is proved by many and varied facts. In the picture-galleries of old families, and on the monumental brasses in the adjacent churches, are often seen types of feature that are still, from time to time, repeated in members of these families. It is matter of common remark that some con- stitutional diseases, such as gout and insanity, after missing a generation, will show themselves in the next. Dr Struthers, in his above-quoted paper on *' Variation in the Number of Fingers and Toes, and of the Phalanges, in Man,'^ gives cases of malformations that were common to grandparent and grandchild, but of which the parent had no trace. M. Girou (as quoted by Mr Sedgwick) says — *' One is often surprised to see lambs black, or spotted with black, born of ewes and rams with white wool, but if one takes the trouble to go back to the origin of this phenomenon, it is found in the an- cestors." Instances still more remarkable, in which the re- moteness of the ancestors copied is very great, are given by Mr Darwin. He points out that in crosses between varieties of the pigeon, there will sometimes re-appear the plumage of the original rock-pigeon, from which these varieties descended; and he instances the faint zebra-like markings occasion- ally traceable in horses, as having probably a like meaning.
HEREDITY. 253 The limitation of Heredity by sex, cannot yet be regarded as established. While in many cases it seems clearly mani- fested; it is in other cases manifested to a very small degreo, if at all. In Mr Sedgwick's essays, already named, will be found evidence implying that there exists some such tendency to limitation, which does or does not show itself distinctly, according to the nature of the organic modification to bo conveyed. But more facts must be collected before any positive conclusion can be reached.
§ 84. A positive explanation of Heredity is not to be expected inthepresent state of Biology. We can look for nothing beyond a simplification of the problem; and a reduction of it to the same category with certain other problems which also admit of hypothetical solution only. If an h}^o thesis which certain other wide-spread phenomena have already thrust upon us, can be shown to render the phenomena of Heredity more in- telligible than they at present seem, w^e shall have reason to entertain it. The applicability of any method of interpreta- tion to two different but allied classes of facts, is evidence of its truth.
The power which organisms display of reproducing lost parts, we saw to be inexplicable except on the assumption that the units of which any organism is built have an innate tendency to arrange themselves into the shape of that organ- ism (§ 65). We inferred that these units must be the pos- sessors of special polarities, resulting from their special struc- tures; and that by the mutual play of their polarities they are compelled to take the form of the species to which they belong. And the instance of the Begonia phyUomaniaca left us no escape from the admission that the ability thus to arrange themselves, is latent in the units contained in every undiffer- entiated cell. Quite in harmony with this conclusion, are certain implications since noticed, respecting the characters of sperm-cells and germ-cells. We saw sundry reasons for rejecting the supposition that these are highly-specialized cells, 254 THE INDUCTIONS OF BIOLOGY.
and for accepting the opposite supposition, that they are cells differing from others rather in being unspecialized. And here the assumption to which we seem driven by the ensemble of the evidence, is, that sperm- cells and germ- cells are essentially nothing more than vehicles, in which are contained small groups of the physiological units in a fit state for obeying their proclivity towards the structural arrangement of the species they belong to. ^ Thus the phenomena of Heredity are seen to assimilate with other phenomena; and the assumption which these other phenomena thrust on us, appears to be equally thrust on us by the phenomena of Heredity. We must con- clude that the likeness of any organism to either parent, is conveyed by the special tendencies of the physiological units derived from that parent. In the fertilized germ we have two groups of physiological units, slightly different in their structures. These slightly- different units, severally multiply at the expense of the nutriment supplied to the unfolding germ — each kind moulding this nutriment into units of its own type. Throughout the process of evolution, the two kinds of units, mainly agreeing in their polarities and in the form which they tend to build themselves into, but having minor differences, work in unison to produce an organism of the species from which they were derived, but work in antagonism to produce copies of their respective parent- organisms. And hence ultimatel}^ results, an organism in which traits of the one are mixed with traits of the other.
If the likeness of offspring to parents is thus determined, it becomes manifest, a priori^ that besides the transmission of generic and specific peculiarities, there will be a transmis- sion of those individual peculiarities which, arising without assignable causes, are classed as *' spontaneous. *' For if the assumption of a special arrangement of parts by an organism, is due to the proclivity of its physiological units towards that arrangement; then the assumption of an arrangement of parts slightly different from that of the species, implies IIKREDITY. 255 physiological units slightly unlike those of tlic species; and these slightly-unlike physiological units, communicated through the medium of sperm-cell or germ-cell, will tend, in the offspring, to build tlicmselves into a structure similarly diverging from the average of the species.
It is not equally manifest, a priori, however, that on this hy- pothesis, alterations of structure caused by alterations of func- tion, must be transmitted to offspring. It is not obvious that change in the form of a part, caused by changed action, in- volves such change in the physiological units throughout the organism, that these, when groups of them are thrown off in the shape of reproductive centres, will unfold into organisms that have this part similarly changed in form. Indeed, when treating of Adaptation (§ 69), we saw that an organ modified by increase or decrease of function, can but slowly so re-act on the system at large, as to bring about those correlative changes required to produce a new equilibrium; and yet only when such new equilibrium has been established, can we ex- pect it to be fillip expressed in the modified physiological units of which the organism is built — only then can we count on a complete transfer of the modification to descendants. ^Nevertheless, that changes of structure caused by changes of action, must also be transmitted, however obscurely, from one generation to another, appears to be a deduction from first principles — or if not a specific deduction, still, a general implication. For if an organism A, has, by any peculiar habit or condition of life, been modified into the form A', it follows inevitably, that all the functions of A', reproductive function included, must be in some degree different from the functions of A. An organism being a combination of rhythmically-acting parts in moving equilibrium, it is im- possible to alter the action and structure of any one part, without causing alterations of action and structure in all the rest; just as no member of the Solar System could be modi- fied in motion or mass, without producing re-arrangements throughout the whole Solar System. And if the organism A, 256 THE INDUCT10>S OF BIOLOGY.
when changed to A', must be changed in all its functions; then the offspring of A' cannot be the same as they would have been had it retained the form A. It involves a denial of the persistence of force to say that A may be changed into A, and may yet beget offspring exactly like those it would have begotten had it not been so changed. That the change in the offspring must, other things equal, be in the same direction as the change in the parent, we may dimly see is implied by the fact, that the change propagated throughout the parental system is a change towards a new state of equilibrium — a change tending to bring the actions of all organs, reproductive included, into harmony with these new actions. Or, bringing the question to its ultimate and simplest form, we may say that as, on the one hand, phy- siological units will, because of their special polarities, build themselves into an organism of a special structure; so, on the other hand, if the structure of this organism is modified by modified function, it will impress some corresponding modification on the structures and polarities of its units. The units and the aggregate must act and re- act on each other. The forces exercised by each unit on the aggregate and by the aggregate on each unit, must ever tend towards a balance. If nothing prevents, the units will mould the aggregate into a form in equilibrium with their pre-existing polarities. If, contrariwise, the aggregate is made by incident actions to take a new form, its forces must tend to re-mould the units into harmony with this new form. And to say that the physiological units are in any degree so re- moulded as to bring their polar forces towards equilibrium with the forces of the modified aggregate, is to say that when separated in the shape of reproductive centres, these units will tend to build themselves up into an aggregate modified in the same di- rection.
CHAPTER IX.
VARIATION.
§ 85. Equally conspicuous with the truth that every organ- ism bears a general likeness to its parents, is the truth that no organism is exactly like either parent. Though similar to both in generic and specific traits, and usually, too, in those traits which distinguish the variety, it diverges in numer- ous traits of minor importance. No two plants are indistin- guishable; and no two animals are without differences. Variation is co- extensive with Heredity.
The degrees of variation have a wide range. There are deviations so small as to be not easily detected; and there are deviations great enough to be called monstrosities. In plants, we may pass from cases of slight alteration in the shape or texture of a leaf, to cases where, instead of a flower with its calyx above the seed-vessel, there is produced a flower with its calyx below the seed-vessel; and while in one animal, there arises a scarcely noticeable unlikeness in the length or colour of the hair, in another, an organ is absent, or a supernumerary organ appears. Though small variations are by far the most general, yet variations of considerable magnitude are not uncommon; and even those variations constituted by additions or suppressions of parts, are not so rare as to be excluded from the list of causes by which organic forms are changed. Cattle without horns are fre- quent. Of sheep there are horned breeds and breeds that 17 258 THE INDUCTIONf^ OF BIOLOGY.
have lost their horns. At one time, there existed in Scot- land a race of pigs with solid feet instead of cleft feet. In pigeons, according to Mr Darwin, " the number of the cau- dal and sacral vertebrae vary; as does the number of the ribs, together with their relative breadth and the presence of processes.'* That variations both small and large which arise without any specific assignable cause, tend to become hereditary, was shown in the last chapter. Indeed the evidence which proves Heredity in its smaller manifestations, is the same evidence which proves Variation; since it is only when there occur vari- ations, that the inheritance of anything beyond the structural peculiarities of the species, can be proved. It remains here, however, to be observed, that the transmission of variations is itself variable; and that it varies both in the direction of decrease and in the direction of increase. An individual trait of one parent, may be so counteracted by the influence of the other parent, that it may not appear in the ofispring; or not being so counteracted, the offspring may possess it, perhaps in an equal degree or perhaps in a less degree; or the off- spring may exhibit the trait in even a still higher degree. Of the illustrations of this, one must suffice. I quote it from the essay by Dr Struthers, referred to in the last chapter.
" The great-great-grandmother, Esther P (who mar- ried A L ), had a sixth little finger on one hand. Of their eighteen children (twelve daughters and six sons), only one (Charles) is known to have had digital variety. We have the history of the descendants of three of the sons, Andrew, Charles, and James.
" (1.) Andrew L had two sons, Thomas and Andrew; and Thomas had two sons all without digital variety. Here we have three successive generations without the variety possessed by the great-grandmother showing itself.
** (2.) James L, who was normal, had two sons and seven daughters, also normal. One of the daughters became Mrs J (one of the informants), and had three daughters VARIATION. 2'59 and five sons, till normal except one of the sons, James J, now aot. 17, who had six fingers on each hand. * * * ** In this branch of the descendants of Esther, we see it passing over two generations and reappearing in one member of the third generation, and now on both hands.
" (3.) Charles L, the only child of Esther who had digital variety, had six fingers on each hand. He had three sons, James, Thomas, and John, all of whom were born with six fingers on each hand, while John has also a sixth toe on one foot. He had also five other sons and four daughters, all of whom were normal.
** (a.) Of the normal children of this, the third generation, the five sons had twelve sons and twelve daughters, and the four daughters have had four sons and four daughters, being the fourth generation, all of whom were normal. A fifth generation in this sub-group consists as yet of only two boys and two girls, who are also normal.
" In this sub-branch, we see the variety of the first gener- ation present in the second, passing over the third and fourth, and also the fifth as far as it has yet gone.
" (b.) James had three sons and two daughters, who are normal.
" (c.) Thomas had four sons and five daughters, who are normal; and has two grandsons, also normal.
" In this sub-branch of the descent, we see the variety of the first generation, showing itself in the second and third, and passing over the fourth, and (as far as it yet exists) the fifth generation.
" {d.) John L (one of the informants) had six fingers, the additional finger being attached on the outer side, as in the case of his brothers James and Thomas. All of them had the additional digits removed. John has also a sixth toe on one foot, situated on the outer side. The fifth and sixth toes have a common proximal phalanx, and a common integument invests the middle and distal phalanges, each having a separate nail.
260 THE INDUCnOXS OF BIOLOGY.
** John L has a son wlio is normal, and a daughter, Jane, who was born with six fingers on each hand and six toes on each foot. The sixth fingers were removed. The sixth toes are not wrapped with the fifth as in her father's case, but are distinct from them. The son has a son and daughter, who, like himself, are normal.
** In this, the most interesting sub-branch of the descent, we see digital increase, which appeared in the first generation on one limb, appearing in the second on two limbs, the hands; in the third on three limbs, the hands and one foot; in the fourth on all the four limbs. There is as yet no fifth generation in uninterrupted transmission of the variety. The variety does not yet occur in any number of the fifth generation of Esther's descendants, which consists, as yet, only of three boys and one girl, whose parents were normal, and of two boys and two girls, whose grandparents were normal. It is not known whether in the case of the greatgreat-grandinother, Esther P, the variety was original or inherited."* § 86. Where there is great uniformity among the mem- bers of a species, the divergences of ofi'spring from the average type, are usually small; but where, among the members of a species, considerable unlikenesses have once been established, unlikenesses among the offspring are fre- quent and great. Wild plants growing in their natural habitats, are uniform over large areas, and maintain from generation to generation like structures; but when cul- tivation has caused appreciable difierences among the mem- bers of any species of plant, extensive and numerous deviations arc apt to arise. Similarly, between wild and domesticated * This remarkable case appears to militate against the conclusion, drawn some few pages back, that the increase of a peculiarity by coincidence of " spontaneous variations " in successive generations, is very improbable; and that the special superiorities of musical composers cannot have thus arisen. The reply is, that the extreme frequency of the occurrence among so narrow a class as that of musical ocmposers, forbids the interpretation thus suggested.
VAllTATTOX, 261 VAllTATTOX, 261 animals of the same species, we see the contrast, that though the homogeneous wild race maintains its type with great per- sistence, the comparatively heterogeneous domestic race fre- quently produces individuals more unlike the average type than the parents are.
Though unlikeness among progenitors is one antecedent of variation, it is by no means the sole antecedent. Were it so, the young ones successively born to the same parents would be alike. If any peculiarity in a new organism were a direct resultant of the structural differences between the two organisms which produced it; then all subsequent new organisms produced by these two, would show the same pecu- liarity. But we know that the successive offspring have differ- ent peculiarities: no two of them are ever exactly alike.
One cause of such structural variation in progeny, is functional variation in parents. Proof of this is given by the fact that, among the progeny of the same parents, there is more difference between those begotten under different con- stitutional states, than between those begotten under the same constitutional state. It is notorious that twins are more nearly alike than children borne in succession. The functional conditions of the parents being the same for twins, but not the same for their brothers and sisters (all other ante- cedents being constant); we h.ave no choice but to admit that variations in the functional conditions of the parents, are the antecedents of those greater unlikenesses which their brothers and sisters exhibit.
Some other antecedent remains, however. The parents being the same, and their constitutional states the same, va- riation, more or less marked, still manifests itself. Plants grown from seeds out of one pod, and animals produced at one birth, are not alike; and sometimes differ considerably. In a litter of pigs or of kittens, we rarely see uniformity of markings; and occasionally, there are important structural contrasts. I have myself recently been shown a litter of Newfoundland puppies, some of which had four digits to 262 THE INDUCTIONS OF BIOLOGY.
their feet, while in otliors, there was present on each hind- foot, what is called the " dew-claw " — a rudimentary fifth digit.
Thus, induction points to three causes of variation, all in action together. We have heterogeneity among progenitors, which, did it act uniformly and alone in generating, by composi- tion of forces, new deviations, would impress such new devia- tions to the same extent on all offspring of the same parents; which it does not. We have functional variation in the pa- rents, which, acting either alone or in combination with the pre- ceding cause, would entail like variations on all young ones simultaneously produced; which it does not. And there is consequently some third cause of variation, yet to be found, which acts along with the structural and functional variations of ancestors and parents.
§ 87. Already, in the last section, there has been implied some relation between variation and the action of external conditions. The above-cited contrast, between the uniformity of wild species and the multiformity of the same species when cultivated or domesticated, thrusts this truth upon us. Respecting the variations of plants, Mr Darwin remarks that ** * sports ' are extremely rare under nature, but far from rare under cultivation.*' Others who have studied the matter assert, that if a species of plant which, up to a certain time, has maintained great uniformity, once has its constitution thoroughly disturbed, it will go on varying indefinitely. Though, in consequence of the remoteness of the periods at which they were domesticated, there is a lack of positive proof that our extremely variable domestic animals have be- come variable under the changed conditions implied by do- mestication, having been previously constant; yet competent judges do not doubt that this has been the case.
Now the constitutional disturbance which precedes varia- tion, can be nothing else than an overthrowing of the pre- established equilibrium of functions. Transferring a plant from forest lands to a ploughed field or a manured garden, is VAIMATION.
altering the balance of forces to which it has been hitherto subject; by supplying it with different proportions of the assimilable matters it requires, and taking away some of the positive impediments to its growth which competing wild plants before offered. An animal taken from woods or plains, where it lived on wild food of its own procuring, and placed under restraint, while artificially supplied with food not quite like what it had before, is an animal subject to new outer ac- tions, to which its inner actions must be re-adjusted. From the general law of equilibration we found it to follow, that '' the maintenance of such a moving equilibrium " as an or- ganism displays, " requires the habitual genesis of internal forces corresponding in number, directions, and amounts, to the external incident forces — as many inner functions, single or combined, as there are single or combined outer ac- tions to be met " {First Principles, § 133); and more recently (§ 27), we have seen that Life itself is " the definite combin- ation of heterogeneous changes, both simultaneous and suc- cessive, in correspondence with external co-existences and sequences.*' Necessarily, therefore, an organism exposed to a permanent change in the arrangement of outer forces, must undergo a permanent change in the arrangement of inner forces. The old equilibrium must be destroyed; and a new equilibrium must be established. There must be func- tional perturbations, ending in a re-adjusted balance of funciions.
If, then, change of conditions is the only known cause by which the original homogeneity of a species is destroyed; and if change of conditions can affect an organism only by altering its functions; it follows that alteration of func- tions is the only known internal cause to which the com- mencement of variation can be ascribed. That such minor functional changes as parents undergo from year to year, are influential on the offspring, we have seen to be proved by the greater unlikeness that exists between children born to the same parents at different times, than exists between 264 IHE INDUCTIONS OF BIOLOGY.
twins. And here we seem forced to conclude, that the larger functional variations produced by greater external changes, are the initiators of those structural variations which, when once commenced in a species, lead by their combinations and antagonisms to multiform results. Whether they are or are not the direct initiators, they must still be the indirect initiators.