Variolation

“ The present is always found to be the key to the past. It is only by employing a similar method that biologists can hope to attack successfully the problems of Variation and Heredity. By actual observation and experiment they must seek to determine how species are being modified around us to-day and find out what characters are transmissible. The nature and causes of Variation and of Heredity will thus in time come to be understood.
It is only within recent years that any attempt has been made to study variations systematically. ”

“ Weismann. According to his conception, variations of germinal origin occur from time to time. By its innate plasticity the several parts of an organism implicated by their association with the varying part are modified in individual life in such away that their modifications cooperate with the germinal variation in producing an adaption of double origin, partly congenital, partly acquired. The organism then waits, so to speak, for a further congenital variation, when a like process of adaptation again occurs ”

“ Now since variation consists as much in meristic change as in alteration in substance or material, there is one great range of problems of causation from which we are as yet entirely cut off. We know nothing of the causation of division, and we have scarcely an observation, experiment or surmise touching the causes by which the meristic processes may be altered.
Of the way in which variations in the substantive composition of organisms are caused we have almost as little real evidence, but we are beginning to know in what such variations must consist. ”

“ Surely, the working of this principle of Correlation indicates a far-sighted Providence of the results, the disastrous monstrosities, that would otherwise have sprung from the law of Variation.
Man’s efforts are considerably limited, moreover, by the law of Reversion. Now, supposing many differing species to be descended, as we maintain, from common ancestors, what ought to be the observable effects of this law? Evidently, we should expect the character of one species now and then to appear in species allied to it, or species of kindred origin to vary in the same manner. ”

“ But if the races crossed differ by more than one factor, and if, further, neither parent is homozygous as regards the factors in which they differ, then we may expect an increase in variability both in F1 and in F2. This is exactly what we observe when the minus race is crossed with either the plus race or its derivative, the mutant race.
If we suppose that the plus race and the minus race differ from each other by certain “modifiers,” we can not suppose that the plus and the mutant races differ by these same modifiers. They differ in some other single respect ”

“ The tendency to mere general variability or plasticity of organisation can certainly be inherited, even from one parent, as has been shown by Gärtner and Kölreuter, in the production of varying hybrids from two species, of which one alone was variable. It is in itself probable that, when an organ has varied in any manner, it will again vary in the same manner, if the conditions which first caused the being to vary remain, as far as can be judged, the same. ”

“ There are two kinds of homogenesis, the simplest of them, probably once universal but now exceptional, being that in which there is no other form of multiplication than one resulting from perpetual spontaneous fission. The rise of distinct sexes was doubtless a step in evolution, and before it took place the formation of new individuals could have arisen only by division of the old, either into two or into many. ”

“ Variability seems to us to be very general, but very limited. The limits however, are distinctly drawn by the struggle for existence. Of course the chance for useful mutations is a very small one. We have seen that the same mutations are as a rule repeated from time to time by the same species. ”

“ Progression and regression are seen to go hand in hand everywhere. No large group and probably even no genus or large species has been evolved without the joint agency of these two great principles. In the mutation-period of the evening-primroses the observed facts give direct support to this conclusion, since some of the new species proved, on closer inspection, to be retrograde varieties, while others manifestly owe their origin to progressive steps. ”

“ After contemplating the several methods by which the multiplication of organisms is carried on—after ranging them under the two heads of Homogenesis, in which the successive generations are similarly produced, and Heterogenesis, in which they are dissimilarly produced—after observing that Homogenesis is nearly always sexual genesis, while Heterogenesis is asexual genesis with occasionally-recurring sexual genesis; we came to the questions—why is it that some organisms multiply in the one way and some in the other? ”

“ No doubt a large amount of variation is truly indefinite, so that many meaningless or useless variations arise, and in one sense it is a mere coincidence if a particular variation turn out to be useful. But there are several directions in which the field of variation appears to be not only limited but defined in a certain direction. Obviously variations depend on the constitution of the varying organism; a modification, whether it be large or small, is a modification of an already definite and limited structure. ”

“ It is essential to distinguish the process of evolution toward the mutation–selection balance from an error catastrophe, as the two are completely different. We thus begin by briefly illustrating the dynamic approach to mutation–selection balance. For any combination of mutation rates and fitnesses in our model, a population will evolve until it reaches a unique equilibrium distribution of A1 and A2. Figure 2 illustrates the evolution of two populations—one that initially consisted entirely of A1 and another that consisted almost entirely of A2. ”

“ How does it happen that some organisms multiply by homogenesis and others by heterogenesis? Why is it that where agamogenesis prevails it is usually from time to time interrupted by gamogenesis? A survey of the facts discloses certain correlations which, if not universal, are too general to be without significance.
Where multiplication is carried on by heterogenesis we find, in numerous cases, that agamogenesis continues as long as the forces which result in growth are greatly in excess of the antagonist forces. ”

“ We cannot declare that Natural Selection has no part in the determination of fixity or variability; nevertheless looking at the whole mass of fact which a study of the incidence of variation provides, I incline to the view that the variability of polymorphic forms should be regarded rather as a thing tolerated than as an element contributing directly to their chances of life ”

“ For every part that is perfectly adapted, although it can fluctuate slightly, yet can never undergo a permanent alteration in the ascending or descending direction because every plus and every minus variation which has attained selective value would be eliminated by personal selection in the course of time. Therefore, a definite direction of variation cannot arise in such cases and we have also reached, as it seems to me, a satisfactory explanation of the constancy of well-adapted species and characters. ”

“ Polymorphism or dimorphism.—By this term I understand the coexistence in the same locality of two or more distinct forms, not connected by intermediate gradations, and all of which are occasionally produced from common parents. These distinct forms generally occur in the female sex only, and the intercrossing of two of these forms does not generate an intermediate race, but reproduces the same forms in varying proportions. I believe it will be found that a considerable number of what have been classed as 6varieties are really cases of polymorphism. ”

“ The transgression is only temporary and a real transition of one form to another does not come within ordinary features of fluctuating variability. Even in the cases of eversporting varieties, where two opposite types are united within one race, and where the succeeding individuals are continually swinging from one extreme to the other, passing through a wide range of intermediate steps, the limits of the variety are as sharply defined and as free from real transgression as in any other form. ”

“ 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. ”

“ The concept of a mutation–selection balance is one of the oldest and most fundamental pillars of population genetics: natural selection increases the frequency of fit variants while mutations introduce unfit variants, giving rise to an equilibrium distribution balanced between these two effects. ”

“ As soon as utility itself is supposed to exercise a determinative influence on the direction of variation, we get an insight into the entire process and into much else besides that has hitherto been regarded as a stumbling-block to the theory of selection, and which did indeed present difficulties that for the moment were insuperable—as, for example, the like-directed variation of a large number of already existing similar parts, seen in the origin of feathers from the scales of reptiles. ”

“ Even in the higher animals various parts are not at all closely related; for one part may be wholly suppressed or rendered monstrous without any other part of the body being affected. But in some cases, when one part varies, certain other parts always, or nearly always, simultaneously vary; they are then subject to the law of correlated variation. Formerly I used the somewhat vague expression of correlation of growth, which may be applied to many large classes of facts. ”

“ For without primary constituents of the germ, whether they are called as I call them, determinants, or something else, germinal selection, or guidance of variation by personal selection, is impossible; for where all units are alike there can be no struggle, no preference of the best. ”

“ 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. ”

“ If we find a relation of exact parallelism to exist between two sets of species in the condition of a certain organ, and the difference so expressed the only one which distinguishes them as sets from each other—if that condition is always the same in each set—we call them two genera: if in any species the condition is variable at maturity, or sometimes the undeveloped condition of the part is persistent and sometimes transitory, the sets characterized by this difference must be united by the systematist, and the whole is called a single genus. ”

“ The elementary species, on the other hand, must have arisen by the production of new qualities, each new acquisition constituting the origin of a new elementary form.
Moreover we have seen, that such improvements and such losses constitute sharp limits between the single unit-forms. Every type, of course, varies around an average, and the extremes of one form may sometimes reach or even overlap those of the nearest allies, but the offspring of the extremes always return to the type. ”

“ The uniformity of the limits of variation within a species may be the result of similarly mutating genes, or of genes derived from hybridization with some exotic influence. Such hereditary property may become fairly well distributed in the course of time thruout any interbreeding population, and if the genes belong to groups of multiple factors controlling single characters in the organism, we have an explanation for the occurrence of every degree of variation between definite limits within a population. ”

“ Mr.Bateson's work, entitled Materials for the Study of Variation, is an important contribution in this direction. He is not content with the knowledge that variability exists but seeks to find out by detailed investigations of particular cases what variations actually do occur in nature. He brings forward evidence to show that there are variations which cannot be arranged in a continuous series—that such variations are therefore discontinuous. ”

“ It seems, indeed, to be a general rule that conspicuous variations occur rarely, and in one individual alone out of many thousands, though all may have been exposed, as far as we can judge, to nearly the same conditions. As the most strongly marked variations graduate insensibly into the most trifling, we are led by the same train of thought to attribute each slight variation much more to innate differences of constitution, however caused, than to the definite action of the surrounding conditions. ”