Part 17

But in Mendelian cases we are concerned primarily not with the product of gametes of blended character, but with the consequences of the union of gametes already discontinuously dissimilar. The existence of pure Mendelian gametes for given characters is perfectly compatible with the existence of blended or mosaic gametes for similar characters elsewhere, but this principle enables us to form a comprehensive and fruitful conception of the relation of the two phenomena to each other. As I also pointed out, through the imperfection of our method which does not yet permit us to _see_ the differentiation among the gametes though we know it exists, we cannot yet as a rule obtain certain proof of the impurity of the gametes (except perhaps in the case of mosaics) as distinct from evidence of imperfect dominance. If however the case be one of a “mule” form, distinct from either parent, and not merely of dominance, there is no _a priori_ reason why even this may not be possible; for we should be able to distinguish the results of breeding first crosses together into _four_ classes: two pure forms, one or more blend or mosaic forms, and “mule” forms. Such a study could as yet only be attempted in simplest cases: for where we are concerned with a compound allelomorph capable of resolution, the combinations of the integral components become so numerous as to make this finer classification practically inapplicable.

But in many cases--perhaps a majority--though by Mendel’s statistical method we can perceive the fluctuations in the numbers of the several products of fertilisation, we shall not know whether abnormalities in the distribution of those products are due to a decline in dominance, or to actual impurity of the gametes. We shall have further to consider, as affecting the arithmetical results, the possibility of departure from the rule that each kind of gamete is produced in equal numbers[164]; also that there may be the familiar difficulties in regard to possible selection and assortative matings among the gametes.

[164] In dealing with cases of decomposition or resolution of compound characters this consideration is of highest importance.

I have now shown how the mosaic and blend-forms are to be regarded in the light of the Mendelian principle. What has Professor Weldon to say in reference to them? His suggestion is definite enough--that a study of ancestry will explain the facts: _how_, we are not told.

In speaking of the need of study of the characters of the _race_ he is much nearer the mark, but when he adds “that is their ancestry,” he goes wide again. When _Telephone_ does not truly divide the antagonistic characters among its germ-cells this fact is in nowise simply traceable to its having originated in a cross--a history it shares with almost all the peas in the market--but to its own peculiar nature. In such a case imperfect dominance need not surprise us.

What we need in all these phenomena is a knowledge of the properties of each race, or variety, as we call it in peas. We must, as I have often pleaded, study the properties of each form no otherwise than the chemist does the properties of his substances, and thus only can we hope to work our way through these phenomena. _Ancestry_ holds no key to these facts; for the same ancestry is common to own brothers and sisters endowed with dissimilar properties and producing dissimilar posterity. To the knowledge of the properties of each form and the laws which it obeys there are no short cuts. We have no periodic law to guide us. Each case must as yet be separately worked out.

We can scarcely avoid mention of a further category of phenomena that are certain to be adduced in opposition to the general truth of the purity of the extracted forms. It is a fact well known to breeders that a highly-bred stock may, unless selections be continued, “degenerate.” This has often been insisted on in regard to peas. I have been told of specific cases by Messrs Sutton and Sons, instances which could be multiplied. Surely, will reply the supporters of the theory of Ancestry, this is simply impurity in the extracted stocks manifesting itself at last. Such a conclusion by no means follows, and the proof that it is inapplicable is obtained from the fact that the “degeneration,” or variation as we should rather call it, need not lead to the production of any proximate ancestor of the selected stock at all, but immediately to a new form, or to one much more remote--in the case of some high class peas, _e.g._, to the form which Mr Sutton describes as “vetch-like,” with short pods, and a very few small round seeds, two or three in a pod. Such plants are recognized by their appearance and are rigorously hoed out every year before seeding.

To appreciate the meaning of these facts we must go back to what was said above on the nature of compound characters. We can perceive that, as Mendel showed, the integral characters of the varieties can be dissociated and re-combined in any combination. More than that; certain integral characters can be resolved into further integral components, by _analytical_ variations. What is taking place in this process of resolution we cannot surmise, but we may liken the consequences of that process to various phenomena of analysis seen elsewhere. To continue the metaphor we may speak of return to the vetch-like type as a _synthetical_ variation: well remembering that we know nothing of any _substance_ being subtracted in the former case or added in the latter, and that the phenomenon is more likely to be primarily one of alteration in arrangement than in substance.

A final proof that nothing is to be looked for from an appeal to ancestry is provided by the fact--of which the literature of variation contains numerous illustrations--that such newly synthesised forms, instead of themselves producing a large proportion of the high class variety which may have been their ancestor for a hundred generations, may produce almost nothing but individuals like themselves. A subject fraught with extraordinary interest will be the determination whether by crossing these newly synthesised forms with their parent, or another pure form, we may not succeed in reproducing a great part of the known series of components afresh. The pure parental form, produced, or extracted, by “analytical” breeding, would not in ordinary circumstances be capable of producing the other components from which it has been separated; but by crossing it with the “synthesised” variety it is not impossible that these components would again reappear. If this can be shown to be possible we shall have entirely new light on the nature of variation and stability.

CONCLUSION.

I trust what I have written has convinced the reader that we are, as was said in opening, at last beginning to move. Professor Weldon declares he has “no wish to belittle the importance of Mendel’s achievement”; he desires “simply to call attention to a series of facts which seem to him to suggest fruitful lines of inquiry.” In this purpose I venture to assist him, for I am disposed to think that unaided he is--to borrow Horace Walpole’s phrase--about as likely to light a fire with a wet dish-clout as to kindle interest in Mendel’s discoveries by his tempered appreciation. If I have helped a little in this cause my time has not been wasted.

In these pages I have only touched the edge of that new country which is stretching out before us, whence in ten years’ time we shall look back on the present days of our captivity. Soon every science that deals with animals and plants will be teeming with discovery, made possible by Mendel’s work. The breeder, whether of plants or of animals, no longer trudging in the old paths of tradition, will be second only to the chemist in resource and in foresight. Each conception of life in which heredity bears a part--and which of them is exempt?--must change before the coming rush of facts.

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_Received as this sheet goes to press:--_

CORRENS, C. Die Ergebnisse der neuesten Bastardforschungen für die Vererbungslehre, _Ber. deut. bot. Ges._, XIX., Generalversammlungs-Heft 1.

---- Ueber den Modus und den Zeitpunkt der Spaltung der Anlagen bei den Bastarden vom Erbsen-Typus, _Bot. Ztg._, 1902, p. 65.