Problems of Genetics

by William Bateson

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. When this variability is sorted out, and is seen to be in part a result of hybridisation, in part a consequence of the persistence of hybrids by parthenogenetic reproduction, a polymorphism due to the continued presence of individuals representing various combinations of Mendelian allelomorphs, partly also the transient effect of alteration in external circumstances, we see how cautious we must be in drawing inferences as to the indefiniteness of specific limits from a bare knowledge that intermediates exist. 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. Lastly, and this is perhaps the most fertile of all the sources of confusion, the impression of the indefiniteness of species is created by the existence of numerous local forms, isolated geographically from each other, forms whose differences may be referable to any one of the categories I have enumerated.

The advance has been from many sides. 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.

If we may once more introduce a physical analogy, the distinctions with which the systematic naturalist is concerned in the study of living things are as multifarious as those by which chemists were confronted in the early days of their science. Diversities due to mechanical mixtures, to allotropy, to differences of temperature and pressure, or to degree of hydration, had all to be severally distinguished before the essential diversity due to variety of chemical constitution stood out clearly, and I surmise that not till a stricter analysis of the diversities of animals and plants has been made on a comprehensive scale, shall we be in a position to declare with any confidence whether there is or is not a natural and physiological distinction between species and variety.

As I have said above, it is in the cases nearest to hand that the problem may be most effectively studied. Comparison between forms from dissimilar situations contributes something; but it is by a close examination of the behaviour, especially the genetic behaviour, of familiar species when living in the presence of their nearest allies that the most direct light on the problem is to be obtained. I cannot understand the attitude of those who, contemplating such facts as this examination elicits, can complacently declare that specific difference is a mere question of degree. With the spread of evolutionary ideas to speak much of the fixity of species has become unfashionable, and yet how striking and inscrutable are the manifestations of that fixity!

Consider the group of species composing the agrestis section of the genus Veronica, namely Tournefortii, agrestis, and polita.

These three grow side by side in my garden, as they do in suitable situations over a vast area of the temperate regions. I have for years noticed them with some care and become familiar with their distinctions and resemblances. Never is there any real doubt as to the identity of any plant. The species show some variability, but I have never seen one which assumed any of the distinguishing features of the others. A glance at the fruits decides at once to which species a plant belongs. I find it impossible to believe that the fixity of these distinctions is directly dependent on their value as aids in the struggle for existence. The mode of existence of the three forms in so far as we can tell is closely similar. By whatever standard we reckon systematic affinity I suppose we shall agree that these species come very near indeed to each other. Bentham even takes the view that polita is a mere variety of agrestis.

Now in such cases as this it has been argued that the specific features of the several types have been separately developed in as many distinct localities, and that their present association is due to subsequent redistribution. Of these Veronicas indeed we know that one, Tournefortii (= Buxbaumii) is as a matter of fact a recent introduction from the east.[8] But this course of argument leads to still further difficulties. For if it is true that the peculiarities of the several species have been perfected and preserved on account of their survival-value to their possessors, it follows that there must be many ways of attaining the same result. But since sufficient adaptation may be ensured in so many ways, the disappearance of the common parent of these forms is difficult to understand. Obviously it must have been a plant very similar in general construction to its modern representatives. Like them it must have been an annual weed, with an organisation conformable to that mode of life. Why then, after having been duly perfected for that existence should it have been entirely superseded in favour of a number of other distinct contrivances for doing the same thing, and—if a gradual transition be predicated—not only by them, but by each intermediate stage between them and the original progenitor? Surely the obvious inference from such facts is that the burden cast upon the theory of gradual selection is far greater than it can bear; that adaptation is not in practice a very close fit, and that the distinctions between these several species of Veronica have not arisen on account of their survival-value but rather because none of their diversities was so damaging as to lead to the extermination of its possessor. When we see these various Veronicas each rigidly reproducing its parental type, all comfortably surviving in competition with each other, are we not forced to the conclusion that tolerance has as much to do with the diversity of species as the stringency of Selection? Certainly these species owe their continued existence to the fact that they are each good enough to live, but how shall we refer the distinctions between them directly or indirectly to the determination of Natural Selection?

The control of Selection is loose while the conformity to specific distinction is often very strict and precise, and no less so even when several closely related species co-exist in the same area and in the same circumstances.

The theory of Selection fails at exactly the point where it was devised to help: Specific distinction.

Let us examine a somewhat different set of facts in the case of another pair of nearly allied species Lychnis diurna and vespertina. The two plants have much in common. Both are dioecious perennials, with somewhat similar flowers, the one crimson, the other white. Each however has its peculiarities which are discernible in almost any part of its structure, whether flower, leaf, fruit or seed, distinctions which would enable a person thoroughly familiar with the plants to determine at once from which species even a small piece had been taken. There is so much resemblance however as readily to support the surmise that the two were mere varieties of one species. Bentham, following Linnaeus, in fact actually makes this suggestion, with what propriety we will afterwards consider. Now this case is typical of many. The two forms have a wide distribution, occurring sometimes separately, sometimes in juxtaposition. L. diurna is a plant of hedgerows and sheltered situations. L. vespertina is common in fields and open spaces, where diurna is hardly ever found; but not rarely vespertina occurs in association with diurna in the places which that plant frequents. In this case I do not doubt that we have to do with organisms of somewhat different aptitudes. That L. vespertina has powers which diurna has not is shown very clearly by the fact that diurna is sometimes entirely absent from areas where vespertina can abound.[9] But in order to understand the true genetic relations of the two plants to each other it is necessary to observe their behaviour when they meet as they not unfrequently do. If the Lychnis population of such a locality be examined it will be found to consist of many undoubted and unmodified diurna, a number—sometimes few, sometimes many—of similarly unmodified vespertina, and an uncertain but usually rather small proportion of plants obviously hybrids between the two. How is it possible to reconcile these facts with the view that specific distinction has no natural basis apart from environmental exigency?