Deletion

“ Nor is it to be wondered at that, with so convenient a mechanism for segregation to hand as cell-division, the progressive differentiation seen during development should often appear to go hand in hand with this process. In recent years, however, evidence has been steadily accumulating to show that this association between cell-division and regional differentiation of the protoplasm in development is a casual one—as casual, and as natural, as the correspondence between cell limits and regional differentiation in the formed tissues. ”

“ It has taken millions of years for the long, slow process of evolution to develop specific adaptations that suit dune dwellers to their environment. There are variations between individuals in every form of life. Mostly these are normal inherited variations, such as height or color. But sometimes sudden variations, called “mutations,” occur through accidental changes in the genes controlling inheritance. These are new characteristics not found in other members of the same species. If the mutation is advantageous it may be passed on, and it is in this way that new life forms slowly develop. ”

“ At any rate, therefore, if differentiation be really the central difficulty in development, it is division which is the essential problem of heredity.
Sir George Darwin and Professor Jeans tell us that "gravitational instability" consequent on the condensation of gases is "the primary agent at work in the actual evolution of the universe," which has led to the division of the heavenly bodies. The greatest advance I can conceive in biology would be the discovery of the nature of the instability which leads to the continual division of the cell. ”

“ Hence whatever we learn about those hereditary elements that stand for defects, we learn just as much about the behavior of the normal partners of those elements. In a word, heredity is not confined to a study of the shuffling of those genes that produce abnormal forms, but is equally concerned with what is going on when normal genes are redistributed. This method of pitting one gene against the other furnishes the only kind of information relating to heredity about which we have precise knowledge. ”

“ With the discovery of this mechanism I venture the opinion that the problem of heredity has been solved. We know how the factors carried by the parents are sorted out to the germ cells. The explanation does not pretend to state how factors arise or how they influence the development of the embryo. But these have never been an integral part of the doctrine of heredity. The problems which they present must be worked out in their own field. So, I repeat, the mechanism of the chromosomes offers a satisfactory solution of the traditional problem of heredity. ”

“ In man and in domesticated animals we find that individuals appear occasionally that are defective in one or another respect. Some of the defects are inherited. Rarely a new one appears that has not been seen before. But the majority of them are reappearances of characters that have been carried under the surface as recessive genes in the germ-plasm. Today we recognize that each of these modifications, if recessive, has first arisen as a mutational change in a single gene before it appeared on the surface as a character by the coming together of two such genes. ”

“ It is this power of spontaneous division which most sharply distinguishes the living from the non-living. In the excellent book dealing with the problems of development, lately published by Mr. Jenkinson a special emphasis is very properly laid on the distinction between the processes of division, and those of differentiation. Too often in discussions of the developmental processes the distinction is obscured. He regards differentiation as the "central difficulty." "Growth and division of the nucleus and the cells," he tells us, are side-issues. ”

“ When we see the phenomenon in this light we realise the obscurity of the problem. The appearance of recessive varieties is comparatively easy to understand. All that is implied is the omission of a constituent. How precisely the omission is effected we cannot suggest, but it is not very difficult to suppose that by some mechanical fault of cell-division a power may be lost. Such variation by unpacking, or analysis of a previously existing complex, though unaccountable, is not inconceivable. But whence come the new dominants? ”

“ The longitudinal division of each chromosome, in which the elements are arranged like beads on a rosary, ensures that the different elements of the whole hereditary substance shall appear in each new cell in exactly the same relative proportion as in the parent cell; just as if two persons had to divide between them a dozen apples of different varieties, and secured perfect equality, not by taking six apples each, but by dividing every apple in two. ”

“ But any variation thus induced in the germ cells would be a factor in each process of the development, and would hence tend to influence every character that appears. Such a hypothesis involves the conception that germ cells contain elements capable of segregation; and this is so. Even if the principle of segregation of characters in inheritance had never been discovered, the principle of segregation of germ-cell elements would still hold, for the two discoveries were made absolutely independently. ”

“ If there are inheritance units, such as determiners or genes, as practically all students of heredity maintain, they must be contained in the germ cells, and it becomes one of the fundamental problems of biology to find out where and what these units are. ”

“ My present task is not to deal with the whole question of heredity, but only with the single although fundamental question—‘How is it that a single cell of the body can contain within itself all the hereditary tendencies of the whole organism?’ I am here leaving out of account the further question as to the forces and the mechanism by which these tendencies are developed in the building-up of the organism. ”

“ In some human cases also the descent is that of a dominant, but irregularities are so frequent that no general rule can yet be perceived. The dominance of such a condition is an exception to the principle that the less-divided is usually dominant to the more-divided, a fact which probably should be interpreted as meaning that divisions are of more than one kind.
Among ordinary somatic divisions, whether of organs, cells, or patterns of differentiation, the control of symmetry is usually manifested. ”

“ If one can control the matings, so that out-breeding takes place each time, the result would undoubtedly be like that in our diagram; but what chance is there for a recessive character, that is neither beneficial nor injurious, if left to itself, to contaminate widely the race with its gene? The answer is that for any one defect there is hardly any chance at all. On the other hand, there is always a possibility that a defect may become widespread despite the chances against each in turn. ”

“ Again, in the higher forms of grouped cells, where something like organisation begins to show itself, the aggregations are not only larger, but the separative process, now carried on by the method of gemmation, no longer wholly destroys the individual. And in fact, this gemmation may be regarded as the form which spontaneous fission must assume in ceasing to be fatal; seeing that gemmation essentially consists in the separation, not into halves, but into a larger part and a smaller part; the larger part continuing to represent the original individual. ”

“ Mutations, sort of unexpected changes, can take place in either the chromosomes or the genes. You see, when this one cell starts dividing, every gene makes a copy of itself; but, sometimes, the copy is a little different from the original. ”

“ Such hypothetical elements of organization must be conceived as distinct from the germ on. They can be shuffled about from one generation to another, and can, therefore, be introduced, removed or replaced in the germ cells. It must be admitted that these conceptions fit certain facts of inheritance in many hybrids fairly well, but the progress of discovery has made necessary the installation of subsidiary hypotheses, so that the most recent conceptions of unit characters are becoming extremely complex, and it would seem as though the system would soon fall of its own weight. ”

“ The recognition of the unit-factors may lead—indeed must lead—to great advances in chemical physiology which without that clue would have been impossible, but in causation the chemical phenomena of heredity must be regarded as secondary to the physical or [Pg 34] mechanical phenomena by which the cells and their constituents are divided and separated. When therefore we speak of the essential phenomena of heredity we mean the mechanics of division, especially, though not, as we shall see, exclusively, of cell-division ”

“ I said that many transfers seem of short-term significance, e.g. because many less-core genes seem to come and go. I did not say that no lateral gene transfers have long-term significance. Surely it would be ridiculous to claim that all lateral transfers shape long-term phylogeny. ”

“ The majority cling to their belief in the Lamarckian factors.
The only safe conclusion to which we can come is that the question as to inheritance of acquired characters has still to be answered.
Evolution depends on Variation and Heredity. The problems of Variation and Heredity are now being attacked from many sides by biologists. It is on their solution that all advance in future must depend. ”

“ Powers are released by mutilation of which in the normal conditions of life no sign can be detected. All who have tried to analyse the phenomena of regeneration are compelled to have recourse to the metaphor of equilibrium, speaking of the normal body as in a state of strain or tension (Morgan) which when disturbed by mutilation results in new division and growth. The forces of division are inacessible to ordinary means of stimulation. ”

“ Can a structural change in the body, induced by some change in use or disuse, or by a change in surrounding influence, affect the germ-cells in such a specific or representative way that the offspring will through its inheritance exhibit, even in a slight degree, the modification which the parent acquired? ”

“ Now, before we examine the numerous, and in some cases most interesting and important, laws of transmission by inheritance, let us make ourselves acquainted with the actual nature of the process. The phenomena of transmission by inheritance are generally looked upon as something quite mysterious, as peculiar processes which cannot be fathomed by natural science, and the causes and actual nature of which cannot be understood. It is precisely in such a case that people very generally assume supernatural influences. ”

“ Whatever the nature of the lethals' action, it can be shown that from among the offspring obtained from certain stocks expected classes are missing, and the absence of these classes can be accounted for on the assumption that there are present mutant factors that follow the Mendelian rule of segregation and which show normal linkage to other factors, but whose only recognizable difference from the normal is the death of those individuals which receive them. ”

“ Those who recognise in the doubtful cases of transmission of a single mutilation present in only one of the parents, proofs of the existence of the disputed operation of heredity, quite forget that the transmission presupposes a very marvellous and extremely complex apparatus which if present at all ought, under certain conditions, to become manifest regularly, and not only in extremely exceptional cases. Nature does not create complex mechanisms in order to leave them unused: they exist by use and for use. ”

“ Every one must have heard of cases of albinism, prickly skin, hairy bodies, &c., appearing in several members of the same family. If strange and rare deviations of structure are truly inherited, less strange and commoner deviations may be freely admitted to be inheritable. Perhaps the correct way of viewing the whole subject, would be, to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly. ”

“ Genesis under every form is a process of negative or positive disintegration; and is thus essentially opposed to that process of integration which is the primary process in individual evolution. Negative disintegration occurs in those cases where, as among the compound Hydrozoa, there is a continuous development of new individuals by budding from the bodies of older individuals; and where the older individuals are thus prevented from growing to a greater size, or reaching a higher degree of integration. ”

“ Heredity, as shown not only in the repetition of the specific structure but in the repetition of ancestral deviations from it, becomes a matter of course; and it falls into unison with the fact that, in various inferior organisms, lost parts can be replaced, and that, in still lower organisms, a fragment can develop into a whole. ”

“ On Cell-division.—We have already said a good deal about the importance of cell-division in ontogeny: it accompanies very many of the processes of organisation in all living beings. But even then, there are the Protozoa, in the morphogenesis of which it does not occur at all, and there have also become known many cases of morphogenesis in higher animals, mostly of the type of regulation, in which cellular division is almost or wholly wanting. ”