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The offspring tends to resemble, sometimes with extraordinary closeness, the parents; this is heredity. This definition omits the cases of (1) alternation of generations, where the offspring resembles a more remote ancestor in the direct line, the alternation being in regular or irregular rhythm; (2) the production of neuters, like their equally neuter uncles and aunts but unlike their (fertile) parents. On the other hand there is almost always, amongst higher forms at any rate, a certain unlikeness as well as a likeness; this is variation. In these two sentences is summed up most of what is absolutely certain respecting these two subjects in spite of the enormous amount of attention which has been devoted to them and the voluminous literature consecrated to them. Some have conceived these two processes as at variance with one another, but this conception is false or at least wholly inadequate. Two methods are employed in studying the processes of heredity, the biometrical method, which seeks to work out the problem by mathematical means, and the method which follows the lines laid down by Abbot Mendel of Brunn, whose long-forgotten observations have led to many very profitable results and give greater hope of a real solution of the difficult questions involved in the subject of heredity than any others which have so far been made public. Particularly do they seem to throw light upon the much-discussed but most imperfectly explained matter known as reversion. Where a unicellular organism divides into two cells it is hard to say which is mother and which is daughter, but there is no difficulty in understanding why both of them closely resemble the cell from which both of them have been derived since both of them are that cell or part of it. This is heredity in its simplest terms. The matter becomes more complicated when the descendant is the offspring of a multicellular organism, even asexually, and infinitely more complicated when ordinary sexual reproduction comes into question. In the asexual case, however, if it could be shown that in the first division of an ovum a certain portion of the substance was set aside for future reproductive purposes, and that this was always the case, the condition would approximate to that of the unicellular organism above mentioned and the heredity would be explained by the fact that the offspring was actually a portion of the original ancestor. Similarly in sexual reproduction, though the matter is more complex, still the offspring would be the result of two ancestors whose reproductive substance had been handed down in the manner indicated above.
This is practically Weismann's "germ-plasm theory". He supposes that each individual consists of two portions, somato-plasm, making up the main portion of the body, and germ-plasm stored away in the sex-gland. This last he believes arises always from germ-plasm, that substance being set aside at the earliest stages of development and finally deposited in the sex-gland, when that organ becomes developed. It is obvious that this theory of a potentially immortal germ-plasm entails great difficulties when the question of variation and especially variation induced by environment, comes into consideration. Moreover, there is no available evidence in higher forms that there is any such setting aside of germinal substance at early stages of development, and all the facts of regeneration are against the theory, as has been pointed out by Hertwig, Weismann's great opponent. If it be true, as it undoubtedly is, that a hydra, cut into several pieces can produce as many new individuals, and a begonia, by cuttings, propagate any number of new plants, it is difficult to see how it can be argued that all the reproductive substance is stored up in one only portion of the animal or plant. Weismann's views, which have undoubtedly exerted great influence on biologists and at one time met with a very large amount of acceptance, have, it must be admitted, failed to meet a great deal of the criticism which has been directed against them, and do not at all hold the position which they occupied some years ago in scientific favour.
Another method of explaining heredity is that which presupposes that fragments from the different portions of the body become aggregated in the sex-cells and thus become the progenitors of the different portions of the offspring. Darwin's theory of "pangenesis" and other similar explanations are of the character, and of them it may be said that they not only rest upon no demonstrable evidence but require so complicated a machinery as to become practically inconceivable. There remains the remarkable theory of "unconscious memory" put forward by Hering, and more recently by Semon and Francis Darwin, and developed in the writings of Samuel Butler. Psychological explanation seems destined to receive more attention in the future than it has in the past. Much doubt remains as to what portion of the cell is the bearer of the hereditary characteristics. Some years ago it was firmly held that these were borne by the nucleus, and further, by the chromosomes of the nucleus alone. Recent experiments have tended to make this theory, if not untenable, at least most doubtful, and it now seems that it may be the nucleus, the protoplasm, or the centrosome, or a combination of any or all of these, which may be held to occupy this position, if indeed it must be held that some definite part of the cell has to be indicated. The inheritance of acquired conditions is a point around which controversy rages and has raged for some time. It may at least be said that Weismann has proved, as far as such a matter can be proved, that mutilations are not inheritable and this may be said in spite of the still doubtful explanation of Brown-Séquard's experiments in connexion with the production of epilepsy in guinea-pigs. Weismann denies the possibility of the inheritance of acquired characters and has invented a cumbrous and, many would say, fanciful explanation to account for variation otherwise. Haeckel, on the contrary, would rather, as he says, believe in the Mosaic theory of creation than doubt the inheritance of acquired conditions.
This matter has often been complicated by the question of inherited disease, which is a wholly different question and of which all that can here be said is that, where it is not a case of bacterial or toxic infection of the germ, it is not the disease which is inherited, but a certain character, or organ, or structure which renders its owner predisposed to the attacks of that disease, should he come in its way. As to the true inheritance of acquired conditions, however, there is, as above indicated, great difference of opinion, the Lamarckians and the so-called neo-Lamarckians holding that in this and in this alone, according to the straiter sect, we have the true explanation of variation and evolution, whilst the Weismannites take up a wholly opposite point of view. It must be admitted that the extremer views of Weismann as to the impossibility of the inheritance of acquired conditions are daily losing ground. The same may be said as to the theory of telegony. It is well known that breeders consider that if a valuable bitch has borne pups to an under-bred dog, she is ruined for breeding purposes, since she is liable at any time to throw ill-bred pups, even though the sire of later litters may be a highly-bred male. The same view is held by horse-breeders. And the condition, which supposes that the maternal organism is, so to speak, infected, by the male congress, is called telegony. The most important argument in its favour is that it is implicitly held by persons whose bread is earned by attention to the laws of inheritance, yet it must be owned that Professor Cossar Ewart's careful experiments, at Penicuik, do not lend authority to the view, and it may perhaps turn out that the true explanation of this puzzling variety of heredity depends on some law of reversion, at present misunderstood, but which may be cleared up by further researches along Mendelian lines. (See IND.)
The subject is well summed up in THOMSON, Heredity (London 1908) written by one belonging to the Weismannite side See also WILSON, The Cell in Development and Inheritance (London and New York, 1896, 1900), the best book on the cellular question which involves so much in the matter of heredity; BATESON Mendel's Principles of Heredity (London 1900); HUTTON The Lesson of Evolution (1907); BUTLER, Life and Habit (London 1878); IDEM, Unconscious Memory (London 1880); BROOKS, The Law of Heredity (Baltimore, 1891); RIBOT L'hérédité (Paris 1873; Eng. tr., London, 1875); MIVART in Dublin Review, CV (1889), pp.269-296; SPENCER, The Inadequacy of Natural Selection in Contemporary Review, LXIII (1893).