Phenotypic variation

“ We have said that the following out of the properties exhibited by physiological species would lead us into difficulties, and at this point they begin to be obvious; for if, as the result of spontaneous variation and of selective breeding, the progeny of a common stock may become separated into groups distinguished from one another by constant, not sexual, morphological characters, it is clear that the physiological definition of species is likely to clash with the morphological definition. ”

“ 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. ”

“ It is however becoming probable that if more knowledge of the chemical and physical structure of organisms is to be attained, the clue will be found through Genetics, and thus that even in the uncoordinated accumulation of facts of Variation we are providing the means of analysis applicable not only to them, but to the problems of normality also.
The only classification that we can yet institute with any confidence among the phenomena of Variation is that which distinguishes on the one hand variations in the processes of division from variations in the nature of the substances divided. ”

“ 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. ”

“ The examples of variation now adduced—and these might have been almost indefinitely increased—will suffice to show that there is hardly an organ or a quality in plants or animals which has not been observed to vary; and further, that whenever any of these variations have been useful to man he has been able to increase them to a marvellous extent by the simple process of always preserving the best varieties to breed from. ”

“ Had the phenomenon of local variation been studied in detail before Darwin wrote, the attempt to make selection responsible for fixity wherever found, could never have been made. The proposition that not only the definiteness of local forms but their variability also is sporadic, can be established by countless illustrations taken from any group of either the animal or the vegetable kingdoms. ”

“ In Darwin's time no serious attempt had been made to examine the manifestations of variability. A vast assemblage of miscellaneous facts could formerly be adduced as seemingly comparable illustrations of the phenomenon "Variation." Time has shown this mass of evidence to be capable of analysis. When first promulgated it produced the impression that variability was a phenomenon generally distributed amongst living things in such a way that the specific divisions must be arbitrary. ”

“ Correlated variation is an important subject for us; for when one part is modified through continued selection, either by man or under nature, other parts of the organisation will be unavoidably modified. From this correlation it apparently follows that, with our domesticated animals and plants, varieties rarely or never differ from each other by some single character alone. ”

“ This last ontogenesis exhibits the phenomena of variation, of reversion, of the predominance of the one or the other parent, etc., and no one is likely to believe that inductive evolutional inquiry alone will ever afford us knowledge of these minute and delicate processes, which, in their bearing on the total resultant development, phylogenesis, are after all the most important of all. ”

“ In fact, we should perhaps more generally conceive of variation as not so much the accomplishment and presentation of one little mark or difference in weight, length, or colour, as the expression of a tendency to vary in a given tissue or organ in a particular way. Thus we are prepared for the rapid extension and dominance of the variation if once it is favoured by selective breeding. ”

“ 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. ”

“ Something has come from the work of systematists, something from cultural experiments, something from the direct study of variation as it appears in nature, but progress is especially due to experimental investigation of heredity. From all these lines of inquiry we get the same answer; that what the naturalists of fifty years ago regarded as variation is not one phenomenon but many, and that what they would have adduced as evidence against the definiteness of species may not in fact be capable of this construction at all. ”

“ It is upon this that the operation of natural selection, that is, personal selection, must depend—that the germ-plasms in which the favourable variation-direction is in the majority are selected for breeding, for it is this and nothing else that natural selection does when it selects the individuals which possess the preferred variations. ”

“ It is demonstrated that among the fluctuating variations there are individuals which are able to transmit their particular variation and give rise by selection to a race, while the majority are not able to hand on their particular conditions to their progeny. Races developed by selection from such variations have not been carried beyond the normal limit of variability of the species. ”

“ Under the head of Variation, in the Family Balanidæ and under the Genus Balanus, I have enlarged on this subject, and have shown that there is scarcely a single external character which is not highly variable in most of the species. As whole groups of specimens often vary in exactly the same manner, it is not easy to exaggerate the difficulty of discriminating species and varieties. ”

“ In all candor we must at least concede that such considerations suggest a genetic descent from the drift period down to the present, and allow time enough—if time is of any account— for variation and natural selection to work out some appreciable results in the way of divergence into races, or even into so-called species. ”

“ But if varieties differing in very slight respects be allowed to cross, such small variation will be destroyed, at least to our senses,—a variation [clearly] just to be distinguished by long legs will have offspring not to be so distinguished. Free crossing great agent in producing uniformity in any breed. Introduce tendency to revert to parent form. ”

“ Whenever and wherever selection is not practised, distinct races are not formed. When any one part of the body or quality is not attended to, it remains either unchanged or varies in a fluctuating manner, whilst at the same time other parts and other qualities may become permanently and greatly modified. But from the tendency to reversion and to continued variability, those parts or organs which are now undergoing rapid improvement through selection, are likewise found to vary much. ”

“ The two germ-plasms differ, I should say, only as regards the determinants of these wing-scales.
We know from the artificial selection to which Man has subjected and still subjects his domesticated animals and useful plants, that any spots and parts of the body which he chooses can be hereditarily altered, if the desired variations which present themselves are always selected for breeding, and that this does not necessarily cause variation in other parts of the body. ”

“ All acknowledge the existence of variations and admit that their proximate source is in the fertilized ovum. All admit that the individual is, through its plasticity, in greater or less degree capable of adaptive modification. Transmissionists contend that the effects of modification are somehow transferred to the germinal substance there to give origin to variations. Selectionists deny this transmission and contend that adaptive [p. 735] variations are independent of adaptive modifications.
Now, what is natural selection, at any rate as understood by the master -- Darwin? ”

“ It is one of Darwin’s great merits to have made use of these observations and to have formulated their results to a large extent as the laws of variation and heredity. As the breeder selects a congenital variation which suits his requirements, and by breeding from the animals (or plants) exhibiting that variation obtains a new breed specially characterized by that variation, so in nature is there a selection amongst all the congenital variations of each generation of a species. ”

“ 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 ”

“ We still know little enough of the mode of occurrence of variation, but we do begin to know something, and if we ask ourselves whether our knowledge, such as it is, conforms at all readily with our former expectations, we cannot with any confidence assert that it does. Among the plants and animals genetically investigated are many illustrations of very striking and distinct varieties. Many of these might readily enough be accepted as species by even the most exacting systematists, and not a few have been so treated in classification ”

“ Any part or organ developed to an extraordinary size or in an extraordinary manner, in comparison with the same part or organ in the allied species, must have gone through an extraordinary amount of modification since the genus arose; and thus we can understand why it should often still be variable in a much higher degree than other parts; for variation is a long-continued and slow process, and natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state. ”

“ Living beings, whether animals or plants, are divisible into multitudes of distinctly definable kinds, which are morphological species. They are also divisible into groups of individuals, which breed freely together, tending to reproduce their like, and are physiological species. Normally resembling their parents, the offspring of members of these species are still liable to vary; and the variation may be perpetuated by selection, as a race, which race, in many cases, presents all the characteristics of a morphological species. ”

“ Conversely, from the accident of collocation or from a misleading resemblance in features we deem essential, forms genetically distinct are often confounded together, and thus the divergence of such forms in their other features, which we declare to be non-essential, passes as an example of variation. ”

“ The species arises neither from the nutrition variety nor from the race; it is always a more advanced variety, and hence species formation is identical with variety formation. Cause for variation and consequently for variety formation is always shown, either when, environment remaining the same, the automatic variation of the idioplasm has advanced so far that the ontogeny is raised to a higher grade of organization and division of labor, or when external stimuli act for a sufficiently long time in a manner not in harmony with the previous adaptation. ”

“ We thus see how a slight but striking variation may at once cause an energetic process of artificial selection, which helps this variation to predominance: a hint for us to be careful in passing judgment upon sexual selection, for the latter also works upon such functionally indifferent but striking variations. ”

“ The new form may appear suddenly, often in a single generation, or gradually, but in either case it is the result of adaptation that is necessarily advantageous, because it is the result of adjustment to controlling physical factors. Origin by adaptation is perhaps only a special kind of origin by variation, but this might be said with equal truth of mutation. New forms resulting from adaptation are like those produced from mutation, in that they appear suddenly as a rule and without the agency of selection. ”

“ Obviously the hereditary diminution of a part is also effected by the simple selection of the individuals in each generation possessing the smallest parts, as is proved, for example, by the tiny bills and feet of numerous breeds of doves. We may assert, therefore, in general terms: a definitely directed progressive variation of a given part is produced by continued selection in that definite direction. ”

“ It is here suggested that the modification as such, is not inherited, but is the condition under which congenital variations are favored and given time to get a hold on the organism, and are thus enabled by degrees to reach the fully adaptive level. When we remember that plastic modification and germinal variation have been working together all along the line of organic evolution, to reach the common goal of adaptation, it is difficult to believe that they have been all along wholly independent of each other. ”