Chapter XIII (2)

On the other hand, with male animals, it is notorious that the secondary sexual characters are more or less completely lost when they are subjected to castration. Thus, if the operation be performed on a young cock, he never, as Yarrell states, crows {52} again; the comb, wattles, and spurs do not grow to their full size, and the hackles assume an intermediate appearance between true hackles and the feathers of the hen. Cases are recorded of confinement alone causing analogous results. But characters properly confined to the female are likewise acquired; the capon takes to sitting on eggs, and will bring up chickens; and what is more curious, the utterly sterile male hybrids from the pheasant and the fowl act in the same manner, "their delight being to watch when the hens leave their nests, and to take on themselves the office of a sitter."[119] That admirable observer Réaumur[120] asserts that a cock, by being long confined in solitude and darkness, can be taught to take charge of young chickens; he then utters a peculiar cry, and retains during his whole life this newly acquired maternal instinct. The many well-ascertained cases of various male mammals giving milk, show that their rudimentary mammary glands retain this capacity in a latent condition.

We thus see that in many, probably in all cases, the secondary characters of each sex lie dormant or latent in the opposite sex, ready to be evolved under peculiar circumstances. We can thus understand how, for instance, it is possible for a good milking cow to transmit her good qualities through her male offspring to future generations; for we may confidently believe that these qualities are present, though latent, in the males of each generation. So it is with the game-cock, who can transmit his superiority in courage and vigour through his female to his male offspring; and with man it is known [121] that diseases, such as hydrocele, necessarily confined to the male sex, can be transmitted through the female to the grandson. Such cases as these offer, as was remarked at the commencement of this chapter, the simplest possible examples of reversion; and they are intelligible on the belief that characters common to the grandparent and grandchild of the same sex are present, though latent, in the intermediate parent of the opposite sex.

The subject of latent characters is so important, as we shall see in a future chapter, that I will give another illustration. {53} Many animals have the right and left sides of their body unequally developed: this is well known to be the case with flat-fish, in which the one side differs in thickness and colour, and in the shape of the fins, from the other; and during the growth of the young fish one eye actually travels, as shown by Steenstrup, from the lower to the upper surface.[122] In most flat-fishes the left is the blind side, but in some it is the right; though in both cases "wrong fishes," which are developed in a reversed manner to what is usual, occasionally occur, and in _Platessa flesus_ the right or left side is indifferently developed, the one as often as the other. With gasteropods or shell-fish, the right and left sides are extremely unequal; the far greater number of species are dextral, with rare and occasional reversals of development, and some few are normally sinistral; but certain species of Bulimus, and, many Achatinellæ,[123] are as often sinistral as dextral. I will give an analogous case in the great Articulate kingdom: the two sides of Verruca[124] are so wonderfully unlike, that without careful dissection it is extremely difficult to recognise the corresponding parts on the opposite sides of the body; yet it is apparently a mere matter of chance whether it be the right or the left side that undergoes so singular an amount of change. One plant is known to me[125] in which the flower, according as it stands on the one or other side of the spike, is unequally developed. In all the foregoing cases the two sides of the animal are perfectly symmetrical at an early period of growth. Now, whenever a species is as liable to be unequally developed on the one as on the other side, we may infer that the capacity for such development is present, though latent, in the undeveloped side. And as a reversal of development occasionally occurs in animals of many kinds, this latent capacity is probably very common.

The best yet simplest instances of characters lying dormant are, perhaps, those previously given, in which chickens and {54} young pigeons, raised from a cross between differently coloured birds, are at first of one colour, but in a year or two acquire feathers of the colour of the other parent; for in this case the tendency to a change of plumage is clearly latent in the young bird. So it is with hornless breeds of cattle, some of which acquire, as they grow old, small horns. Purely bred black and white bantams, and some other fowls, occasionally assume, with advancing years, the red feathers of the parent-species. I will here add a somewhat different case, as it connects in a striking manner latent characters of two classes. Mr. Hewitt[126] possessed an excellent Sebright gold-laced hen bantam, which, as she became old, grew diseased in her ovaria, and assumed male characters. In this breed the males resemble the females in all respects except in their combs, wattles, spurs, and instincts; hence it might have been expected that the diseased hen would have assumed only those masculine characters which are proper to the breed, but she acquired, in addition, well-arched tail sickle-feathers quite a foot in length, saddle-feathers on the loins, and hackles on the neck,--ornaments which, as Mr. Hewitt remarks, "would be held as abominable in this breed." The Sebright bantam is known[127] to have originated about the year 1800 from a cross between a common bantam and a Polish fowl, recrossed by a hen-tailed bantam, and carefully selected; hence there can hardly be a doubt that the sickle-feathers and hackles which appeared in the old hen were derived from the Polish fowl or common bantam; and we thus see that not only certain masculine characters proper to the Sebright bantam, but other masculine characters derived from the first progenitors of the breed, removed by a period of above sixty years, were lying latent in this hen-bird, ready to be evolved as soon as her ovaria became diseased.

From these several facts it must be admitted that certain characters, capacities, and instincts may lie latent in an individual, and even in a succession of individuals, without our being able to detect the least signs of their presence. We have {55} already seen that the transmission of a character from the grandparent to the grandchild, with its apparent omission in the intermediate parent of the opposite sex, becomes simple on this view. When fowls, pigeons, or cattle of different colours are crossed, and their offspring change colour as they grow old, or when the crossed turbit acquired the characteristic frill after its third moult, or when purely-bred bantams partially assume the red plumage of their prototype, we cannot doubt that these qualities were from the first present, though latent, in the individual animal, like the characters of a moth in the caterpillar. Now, if these animals had produced offspring before they had acquired with advancing age their new characters, nothing is more probable than that they would have transmitted them to some of their offspring, which in this case would in appearance have received such characters from their grandparents or more distant progenitors. We should then have had a case of reversion, that is, of the reappearance in the child of an ancestral character, actually present, though during youth completely latent, in the parent; and this we may safely conclude is what occurs with reversions of all kinds to progenitors however remote.

This view of the latency in each generation of all the characters which appear through reversion, is also supported by their actual presence in some cases during early youth alone, or by their more frequent appearance and greater distinctness at this age than during maturity. We have seen that this is often the case with the stripes on the legs and faces of the several species of the horse-genus. The Himalayan rabbit, when crossed, sometimes produces offspring which revert to the parent silver-grey breed, and we have seen that in purely bred animals pale-grey fur occasionally reappears during early youth. Black cats, we may feel assured, would occasionally produce by reversion tabbies; and on young black kittens, with a pedigree[128] known to have been long pure, faint traces of stripes may almost always be seen which afterwards disappear. Hornless Suffolk cattle occasionally produce by reversion horned animals; and Youatt[129] asserts that even in hornless individuals {56} "the rudiment of a horn may be often felt at an early age."

No doubt it appears at first sight in the highest degree improbable that in every horse of every generation there should be a latent capacity and tendency to produce stripes, though these may not appear once in a thousand generations; that in every white, black, or other coloured pigeon, which may have transmitted its proper colour during centuries, there should be a latent capacity in the plumage to become blue and to be marked with certain characteristic bars; that in every child in a six-fingered family there should be the capacity for the production of an additional digit; and so in other cases. Nevertheless there is no more inherent improbability in this being the case than in a useless and rudimentary organ, or even in only a tendency to the production of a rudimentary organ, being inherited during millions of generations, as is well known to occur with a multitude of organic beings. There is no more inherent improbability in each domestic pig, during a thousand generations, retaining the capacity and tendency to develop great tusks under fitting conditions, than in the young calf having retained for an indefinite number of generations rudimentary incisor teeth, which never protrude through the gums.

I shall give at the end of the next chapter a summary of the three preceding chapters; but as isolated and striking cases of reversion have here been chiefly insisted on, I wish to guard the reader against supposing that reversion is due to some rare or accidental combination of circumstances. When a character, lost during hundreds of generations, suddenly reappears, no doubt some such combination must occur; but reversions may be constantly observed, at least to the immediately preceding generations, in the offspring of most unions. This has been universally recognised in the case of hybrids and mongrels, but it has been recognised simply from the difference between the united forms rendering the resemblance of the offspring to their grandparents or more remote progenitors of easy detection. Reversion is likewise almost invariably the rule, as Mr. Sedgwick has shown, with certain diseases. Hence we must conclude that a tendency to this peculiar form of transmission is an integral part of the general law of inheritance. {57}

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_Monstrosities._--A large number of monstrous growths and of lesser anomalies are admitted by every one to be due to an arrest of development, that is to the persistence of an embryonic condition. If every horse or ass had striped legs whilst young, the stripes which occasionally appear on these animals when adult would have to be considered as due to the anomalous retention of an early character, and not as due to reversion. Now, the leg-stripes in the horse-genus, and some other characters in analogous cases, are apt to occur during early youth and then to disappear; thus the persistence of early characters and reversion are brought into close connexion.

But many monstrosities can hardly be considered as the result of an arrest of development; for parts of which no trace can be detected in the embryo, but which occur in other members of the same class of animals or plants, occasionally appear, and these may probably with truth be attributed to reversion. For instance: supernumerary mammæ, capable of secreting milk, are not extremely rare in women; and as many as five have been observed. When four are developed, they are generally arranged symmetrically on each side of the chest; and in one instance a woman (the daughter of another with supernumerary mammæ) had one mamma, which yielded milk, developed in the inguinal region. This latter case, when we remember the position of the mammæ in some of the lower animals on both the chest and inguinal region, is highly remarkable, and leads to the belief that in all cases the additional mammæ in woman are due to reversion. The facts given in the last chapter on the tendency in supernumerary digits to regrowth after amputation, indicate their relation to the digits of the lower vertebrate animals, and lead to the suspicion that their appearance may in some manner be connected with reversion. But I shall have to recur, in the chapter on pangenesis, to the abnormal multiplication of organs, and likewise to their occasional transposition. The occasional development in man of the coccygeal vertebræ into a short and free tail, though it thus becomes in one sense more perfectly developed, may at the same time be considered as an arrest of development, and as a case of reversion. The greater frequency of a monstrous kind of proboscis in the pig than in any other mammal, considering the position of the pig {58} in the mammalian series, has likewise been attributed, perhaps truly, to reversion.[130]

When flowers which are properly irregular in structure become regular
or peloric, the change is generally looked at by botanists as a return
to the primitive state. But Dr. Maxwell Masters,[131] who has ably
discussed this subject, remarks that when, for instance, all the sepals
of a Tropæolum become green and of the same shape, instead of being
coloured with one alone prolonged into a spur, or when all the petals
of a Linaria become simple and regular, such cases may be due merely to
an arrest of development; for in these flowers all the organs during
their earliest condition are symmetrical, and, if arrested at this
stage of growth, they would not become irregular. If, moreover, the
arrest were to take place at a still earlier period of development, the
result would be a simple tuft of green leaves; and no one probably
would call this a case of reversion. Dr. Masters designates the cases
first alluded to as regular peloria; and others, in which all the
corresponding parts assume a similar form of irregularity, as when all
the petals in a Linaria become spurred, as irregular peloria. We have
no right to attribute these latter cases to reversion, until it can be
shown to be probable that the parent-form, for instance, of the genus
Linaria had had all its petals spurred; for a change of this nature
might result from the spreading of an anomalous structure, in
accordance with the law, to be discussed in a future chapter, of
homologous parts tending to vary in the same manner. But as both forms
of peloria frequently occur on the same individual plant of the
Linaria,[132] they probably stand in some close relation to each other.
On the doctrine that peloria is simply the result of an arrest of
development, it is difficult to understand how an organ arrested at a
very early period of growth should acquire its full functional
perfection;--how a petal, supposed to be thus arrested, should acquire
its brilliant colours, and serve as an envelope to the flower, or a
stamen produce efficient pollen; yet this occurs with many peloric
flowers. That pelorism is not due to mere chance variability, but
either to an arrest of development or to reversion, we may infer from
an observation made by Ch. Morren,[133] namely, that families which
have irregular flowers often "return by these monstrous growths to
their regular form; whilst we never see a regular flower realise the
structure of an irregular one."

Some flowers have almost certainly become more or less completely
peloric through reversion. _Corydalis tuberosa_ properly has one of its
two nectaries colourless, destitute of nectar, only half the size of
the other, and {59} therefore, to a certain extent, in a rudimentary
state; the pistil is curved towards the perfect nectary, and the hood,
formed of the inner petals, slips off the pistil and stamens in one
direction alone, so that, when a bee sucks the perfect nectary, the
stigma and stamens are exposed and rubbed against the insect's body. In
several closely allied genera, as in Dielytra, &c., there are two
perfect nectaries, the pistil is straight, and the hood slips off on
either side, according as the bee sucks either nectary. Now, I have
examined several flowers of _Corydalis tuberosa_, in which both
nectaries were equally developed and contained nectar; in this we see
only the redevelopment of a partially aborted organ; but with this
redevelopment the pistil becomes straight, and the hood slips off in
either direction; so that these flowers have acquired the perfect
structure, so well adapted for insect agency, of Dielytra and its
allies. We cannot attribute these coadapted modifications to chance, or
to correlated variability; we must attribute them to reversion to a
primordial condition of the species.

The peloric flowers of Pelargonium have their five petals in all
respects alike, and there is no nectary; so that they resemble the
symmetrical flowers of the closely allied Geranium-genus; but the
alternate stamens are also sometimes destitute of anthers, the
shortened filaments being left as rudiments, and in this respect they
resemble the symmetrical flowers of the closely allied genus, Erodium.
Hence we are led to look at the peloric flowers of Pelargonium as
having probably reverted to the state of some primordial form, the
progenitor of the three closely related genera of Pelargonium,
Geranium, and Erodium.

In the peloric form of _Antirrhinum majus_, appropriately called the
"_Wonder_," the tubular and elongated flowers differ wonderfully from
those of the common snapdragon; the calyx and the mouth of the corolla
consist of six equal lobes, and include six equal instead of four
unequal stamens. One of the two additional stamens is manifestly formed
by the development of a microscopically minute papilla, which may be
found at the base of the upper lip of the flower in all common
snapdragons, at least in nineteen plants examined by me. That this
papilla is a rudiment of a stamen was well shown by its various degrees
of development in crossed plants between the common and peloric
Antirrhinum. Again, a peloric _Galeobdolon luteum_, growing in my
garden, had five equal petals, all striped like the ordinary lower lip,
and included five equal instead of four unequal stamens; but Mr. R.
Keeley, who sent me this plant, informs me that the flowers vary
greatly, having from four to six lobes to the corolla, and from three
to six stamens.[134] Now, as the members of the two great families to
which the Antirrhinum and Galeobdolon belong are properly pentamerous,
with some of the parts confluent and others suppressed, we ought not to
look at the sixth stamen and the sixth lobe to the corolla in either
case as due to reversion, any more than the additional petals in double
flowers in these same two families. But the case is different with the
fifth stamen in the peloric Antirrhinum, which {60} is produced by the
redevelopment of a rudiment always present, and which probably reveals
to us the state of the flower, as far as the stamens are concerned, at
some ancient epoch. It is also difficult to believe that the other four
stamens and the petals, after an arrest of development at a very early
embryonic age, would have come to full perfection in colour, structure,
and function, unless these organs had at some former period normally
passed through a similar course of growth. Hence it appears to me
probable that the progenitor of the genus Antirrhinum must at some
remote epoch have included five stamens and borne flowers in some
degree resembling those now produced by the peloric form.

Lastly, I may add that many instances have been recorded of flowers,
not generally ranked as peloric, in which certain organs, normally few
in number, have been abnormally augmented. As such an increase of parts
cannot be looked at as an arrest of development, nor as due to the
redevelopment of rudiments, for no rudiments are present, and as these
additional parts bring the plant into closer relationship with its
natural allies, they ought probably to be viewed as reversions to a
primordial condition.

These several facts show us in an interesting manner how intimately certain abnormal states are connected together; namely, arrests of development causing parts to become rudimentary or to be wholly suppressed,--the redevelopment of parts at present in a more or less rudimentary condition,--the reappearance of organs of which not a vestige can now be detected,--and to these may be added, in the case of animals, the presence during youth, and subsequent disappearance, of certain characters which occasionally are retained throughout life. Some naturalists look at all such abnormal structures as a return to the ideal state of the group to which the affected being belongs; but it is difficult to conceive what is meant to be conveyed by this expression. Other naturalists maintain, with greater probability and distinctness of view, that the common bond of connection between the several foregoing cases is an actual, though partial, return to the structure of the ancient progenitor of the group. If this view be correct, we must believe that a vast number of characters, capable of evolution, lie hidden in every organic being. But it would be a mistake to suppose that the number is equally great in all beings. We know, for instance, that plants of many orders occasionally become peloric; but many more cases have been observed in the Labiatæ and Scrophulariaceæ than in any other order; and in one genus of the Scrophulariaceæ, namely Linaria, no less {61} than thirteen species have been described in a peloric condition.[135] On this view of the nature of peloric flowers, and bearing in mind what has been said with respect to certain monstrosities in the animal kingdom, we must conclude that the progenitors of most plants and animals, though widely different in structure, have left an impression capable of redevelopment on the germs of their descendants.

The fertilised germ of one of the higher animals, subjected as it is to so vast a series of changes from the germinal cell to old age,--incessantly agitated by what Quatrefages well calls the _tourbillon vital_,--is perhaps the most wonderful object in nature. It is probable that hardly a change of any kind affects either parent, without some mark being left on the germ. But on the doctrine of reversion, as given in this chapter, the germ becomes a far more marvellous object, for, besides the visible changes to which it is subjected, we must believe that it is crowded with invisible characters, proper to both sexes, to both the right and left side of the body, and to a long line of male and female ancestors separated by hundreds or even thousands of generations from the present time; and these characters, like those written on paper with invisible ink, all lie ready to be evolved under certain known or unknown conditions.

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