Reversion (biology)

“ The principle of reversion is the touchstone of the fitness for survival of blood variations normal in character. If they withstand for indefinite generations the tendency to be supplanted by older and more deeply fixed characters, their fitness is proved, and at the same time they acquire prepotency for strong maintenance against fresh deviations. ”

“ It must in the first place be taken into consideration that the phyletic development of species need not by any means always take place by advancement. We have indeed many cases of retrogressive development, although in a somewhat different sense, as with parasites and those forms which have degenerated from free locomotion to a sedentary mode of life.243 I do not confuse this kind of retrogressive development, arising from the arrest of certain organs and584 systems of organs, with true reversion. ”

“ Summary on the proximate causes leading to Reversion.—When purely-bred animals or plants reassume long-lost characters,—when the common ass, for instance, is born with striped legs, when a pure race of black or white pigeons throws a slaty-blue bird, or when a cultivated heartsease with large and rounded flowers produces a seedling with small and elongated flowers,—we are quite unable to assign any proximate cause. When animals run wild, the tendency to reversion, which, though it has been greatly exaggerated, no doubt exists, is sometimes to a certain extent intelligible. ”

“ Reversion, in the ordinary sense of the word, acts so incessantly, that it evidently forms an essential part of the general law of inheritance. It occurs with beings, however propagated, whether by buds or seminal generation, and sometimes may be observed with advancing age even in the same individual. ”

“ Stronger confirmation of our hypothesis, short of a full and unequivocal confession of its validity, we are utterly unable to conceive. Are we not, after this, justified in ascribing to reversion every favorable modification which has arisen or may arise?
Having thus furnished full warrant for assuming degeneration and subsequent favorable reversion, and for alleging the complete gratuitousness of the converse assumption of evolution, let us turn our attention to the grand principle of natural selection. ”

“ On the hypothesis of reversion, correlation is perfectly explicable. The supposition of reversion necessarily involves the conclusion that all the features of the species coexisted in each individual, saving, of course, the characters peculiar to the opposite sex. The perfect organism, then, is a balance of all the parts. ”

“ 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. ”

“ From what we see of the power and scope of reversion, both in pure races, and when varieties or species are crossed, we may infer that characters of almost every kind are capable of reappearing after having been lost for a great length of time. ”

“ Why does an organism as soon as it is established at the fertilisation of the ovum enter upon a cycle of transformations which never cease until death puts an end to them? In other words what is the meaning of that cycle of changes which all organisms present in a greater or less degree and which constitute the very essence of life? It is impossible to give an answer to this question so long as we remain within the precincts of Biology—and it is not my present purpose to penetrate beyond those precincts into the realms of philosophy. ”

“ In the foregoing description of the processes of formation of domesticated varieties, we have assumed reversion as the cause of modifications. We have occasion now to speak of a process which implies a cause that is not reversion. Varieties are formed, and disproportionate development and divergence of character effected, by man's continuing the process of degeneration commenced under nature. ”

“ That all organic beings, including man, present many modifications of structure which are of no service to them at present, nor have been formerly, is, as I can now see, probable. We know not what produces the numberless slight differences between the individuals of each species, for reversion only carries the problem a few steps backwards ”

“ Nor can we account in all cases for the appearance through bud-variation of new characters by the principle of reversion to long-lost characters. He who wishes to judge how far the conditions of life directly cause each particular variation ought to reflect well on the cases immediately to be given. ”

“ Unless we choose or invent some new term, we must define adaptations apart from all questions of evolution as any structures or characters in an organism which can be shown either by their mere presence, or by their active function, to be either useful or necessary to the animal's existence. We must be on our guard against assuming that the word 'adaptation' implies any particular theory or conclusion concerning the method and process by which adaptations have arisen in the course of evolution. ”

“ Deductive interpretation of the phenomena of Repair, is by no means so easy. The tendency displayed by an animal organism, as well as by each of its organs, to return to a state of integrity by the assimilation of new matter, when it has undergone the waste consequent on activity, is a tendency which is not manifestly deducible from first principles ”

“ It should also be remembered that many characters lie latent in all organic beings, ready to be evolved under fitting conditions; and in breeds modified within recent times, the tendency to reversion is particularly strong. But the antiquity of some of our breeds clearly proves that they remain nearly constant as long as their conditions of life remain the same. ”

“ 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 ”

“ It is very advisable to distinguish between the mere impulses which are able to produce sudden reversion, and between actual transforming causes which result directly or indirectly in the remodelling of a species. Thus, it is conceivable à priori that reversion may occur by the action of an inciting cause having nothing to do with the origin of the phyletically older form. ”

“ First Difference, Reversion.—The strong tendency of artificial varieties to reversion, even during the process of formation, and especially their complete reversion to the original type if the hand of man be withdrawn—i. e., if left to themselves, or become wild—is supposed to show an essential difference between such varieties, however extreme, and true species—is supposed, in fact, to prove an indestructible permanency of specific types. Nature disowns these artificial forms, and as it were brands them with bastardy. Not only so, she strives ever to destroy them. ”

“ In my abstract (72/1. "The Origin of Species.") I give only a paragraph on the general case of reversion, though I enter in detail on some cases of reversion of a special character. I have not as yet put all my facts on this subject in mass, so can come to no definite conclusion. But as single characters may revert, I must say that I see no improbability in several reverting. As I do not believe any well-founded experiments or facts are known, each must form his opinion from vague generalities. ”

“ It is here suggested that persistent modification through many generations, though not transmitted to the germ, nevertheless affords the opportunity for the occurrence of germinal variation of like nature. 7. Under constant conditions of life, though variations in many directions are occurring in the organisms which have reached harmonious adjustment: to these conditions, yet natural selection eliminates all those which are of such amount as to be disadvantageous, and thus acts as a check on all variations, repressing them to within narrow limits. ”

“ The simple brain of a microcephalous idiot, in as far as it resembles that of an ape, may in this sense be said to offer a case of reversion. There are other cases which come more strictly under our present heading of reversion. Certain structures, regularly occurring in the lower members of the group to which man belongs, occasionally make their appearance in him, though not found in the normal human embryo; or, if present in the normal human embryo, they become developed in an abnormal manner, though this manner of development is proper to the lower members of the same group. ”

“ In answer to these two objections we must bear in mind the evidence which has been advanced, showing that at least in a large number of cases the power of regrowth is a localized faculty, acquired for the sake of repairing special injuries to which each particular creature is liable; and, in the case of buds or fissiparous generation, for the sake of quickly multiplying the organism at a period of life when it can be supported in large numbers. ”

“ The chief evidences of the theory of descent are to be found in homologies, in the correspondences of organs and functions, as revealed by comparative anatomy and physiology, in the recapitulation revealed by embryology, in the structure of parasites, in rudimentary organs and reversions to earlier stages, in the distribution of animals and plants, and in the possibility of still transforming, at least to a slight extent, one species into another, by experimental breeding.
Transformation and differentiation go on in nature as a vast, ceaseless, but blind process of selection. ”

“ In these normal cases the regeneration depends on the existence of special formative layers or groups of cells, and must be regarded in each case as a special adaptation, with individual limitations and peculiarities, rather than as a mere exhibition of the fundamental power of growth and reproduction displayed by living substance. Many tissues, even in the highest animals, are capable of replacing an abnormal loss of substance. ”

“ The structure and functions of organized bodies are subject to continual alteration. The changes of nutrition and growth, which are constantly taking place in the tissues render them at the same time the seat of repair and waste, of renovation and decomposition, of life and death. The plant germinates and blossoms, then withers and decays; animal life, in like manner, comes into being, grows to maturity, fades, and dies. It is, therefore, essential to the perpetuation of life, that new organisms be provided to take the place of those which are passing out of existence. ”

“ Hence, in parasites and some forms of life which live under simple conditions, we have the phenomena of degeneration, or a passage from a more complex to a more simple condition.
Revolution in organic life is the destruction of one organism or group of organisms, and the replacement in its stead of a wholly different organism or group of organisms. During hard times there may be much revolution, or replacement of one set of organic forms by another set of organic forms. ”

“ This selection exhibits far-reaching analogies between the artificial and the natural processes, and is in both cases of the very highest importance. In nature the dying out of unfit mutations is the result of the great struggle for life. ”

“ Where the change is too great, or where an organ has been modified cumulatively in some one direction, until it has reached a development too seriously out of harmony with the habits of the organism taken collectively, then the organism holds itself excused from further effort, throws up the whole concern, and takes refuge in the liquidation and reconstruction of death. ”

“ We have then to learn whether the altered conditions of life, by forcing an organism to adopt new habits, can by such means lead directly, and not indirectly through natural selection, to the transformation of the species; or whether the effects of increased or diminished use of certain parts, implied by the new habits, are restricted to the individual itself, and therefore powerless to effect any direct modification of the species. ”

“ Indeed, when treating of Adaptation, we saw that an organ modified by increase or decrease of function can but slowly so react 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 expect it to be fully 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."... ”

“ The cenogenetic alterations or distortions of the original palingenetic course of development take the form, as a rule, of a gradual displacement of the phenomena, which is slowly effected by adaptation to the changed conditions of embryonic existence during the course of thousands of years. ”

“ Thus, a simple variation and adaptation has been produced, and if the same organism continues in this position, its bottom cells will estrange themselves from their original function, while those on the top will convey the food into the interior, where a cavity will be formed, ultimately with a permanent opening, the primitive gut and mouth, both very different from the 'foot.'
Thus, by adaptation and variation the organism acquires new functions, organs, features, and it gives up and eventually loses others. Its offspring is like it. Like produces like. This is the principle of heredity. ”