Part 3

A. In regard to cotyledon colour: Preliminary, 137. Xenia, 139. (1) Gärtner’s cases, 141. (2) Seton’s case, 143. (3) Tschermak’s exceptions, 145. (3a) Buchsbaum case, 145. (3b) Telephone cases, 146. (3c) Couturier cases, 147.

B. Seed-coats and Shapes. 1. Seed-coats, 148. 2. Seed-shapes: (a) Rimpau’s cases, 150. (b) Tschermak’s cases, 152. 3. Other phenomena, especially regarding seed-shapes, in the case of “grey” peas. Modern evidence, 153.

C. Evidence of Knight and Laxton, 158.

D. Miscellaneous cases in other plants and animals:

1. Stocks (Matthiola). Hoariness, 169. Flower-colour, 170.

2. Datura, 172.

3. Colours of Rats and Mice, 173.

V. Professor Weldon’s quotations from Laxton, 178.

Illustration from Primula sinensis, 182.

VI. The Argument built on exceptions, 183.

Ancestry and Dominance, 185.

Ancestry and purity of germ-cells, 193.

The value of the appeal to Ancestry, 197.

VII. The question of absolute purity of germ-cells, 201.

Conclusion, 208.

ERRATA.

p. 22, par. 3, line 2, for “falls” read “fall.” p. 63, line 12, for “AabbC” read “AaBbc.” p. 66, in heading, for “OF HYBRIDS” read “OF THE HYBRIDS.”

Note to p. 125. None of the yellow seeds produced by Laxton’s Alpha germinated, though almost all the green seeds sown gave healthy plants. The same was found in the case of Express, another variety which bore some yellow seeds. In the case of Blue Peter, on the contrary, the yellow seeds have grown as well as the green ones. Few however were wholly yellow. Of nine yellow seeds produced by crossing green varieties together (p. 131), six did not germinate, and three which did gave weak and very backward plants. Taken together, this evidence makes it scarcely doubtful that the yellow colour in these cases was pathological, and almost certainly due to exposure after ripening.

THE PROBLEMS OF HEREDITY AND THEIR SOLUTION.

The first half of this paper is reprinted with additions and modifications from the Journal of the Royal Horticultural Society, 1900, vol. XXV., parts 1 and 2. Written almost immediately after the rediscovery of Mendel, it will be seen to be already in some measure out of date, but it may thus serve to show the relation of the new conceptions to the old.

An exact determination of the laws of heredity will probably work more change in man’s outlook on the world, and in his power over nature, than any other advance in natural knowledge that can be clearly foreseen.

There is no doubt whatever that these laws can be determined. In comparison with the labour that has been needed for other great discoveries we may even expect that the necessary effort will be small. It is rather remarkable that while in other branches of physiology such great progress has of late been made, our knowledge of the phenomena of heredity has increased but little; though that these phenomena constitute the basis of all evolutionary science and the very central problem of natural history is admitted by all. Nor is this due to the special difficulty of such inquiries so much as to general neglect of the subject.

It is in the hope of inducing others to follow these lines of investigation that I take the problems of heredity as the subject of this lecture to the Royal Horticultural Society.

No one has better opportunities of pursuing such work than horticulturists and stock breeders. They are daily witnesses of the phenomena of heredity. Their success also depends largely on a knowledge of its laws, and obviously every increase in that knowledge is of direct and special importance to them.

The want of systematic study of heredity is due chiefly to misapprehension. It is supposed that such work requires a lifetime. But though for adequate study of the complex phenomena of inheritance long periods of time must be necessary, yet in our present state of deep ignorance almost of the outline of the facts, observations carefully planned and faithfully carried out for even a few years may produce results of great value. In fact, by far the most appreciable and definite additions to our knowledge of these matters have been thus obtained.

There is besides some misapprehension as to the kind of knowledge which is especially wanted at this time, and as to the modes by which we may expect to obtain it. The present paper is written in the hope that it may in some degree help to clear the ground of these difficulties by a preliminary consideration of the question, How far have we got towards an exact knowledge of heredity, and how can we get further?

Now this is pre-eminently a subject in which we must distinguish what we can do from what we want to do. We want to know the whole truth of the matter; we want to know the physical basis, the inward and essential nature, “the causes,” as they are sometimes called, of heredity: but we want also to know the laws which the outward and visible phenomena obey.

Let us recognise from the outset that as to the essential nature of these phenomena we still know absolutely nothing. We have no glimmering of an idea as to what constitutes the essential process by which the likeness of the parent is transmitted to the offspring. We can study the processes of fertilisation and development in the finest detail which the microscope manifests to us, and we may fairly say that we have now a considerable grasp of the visible phenomena; but of the nature of the physical basis of heredity we have no conception at all. No one has yet any suggestion, working hypothesis, or mental picture that has thus far helped in the slightest degree to penetrate beyond what we see. The process is as utterly mysterious to us as a flash of lightning is to a savage. We do not know what is the essential agent in the transmission of parental characters, not even whether it is a material agent or not. Not only is our ignorance complete, but no one has the remotest idea how to set to work on that part of the problem. We are in the state in which the students of physical science were, in the period when it was open to anyone to believe that heat was a material substance or not, as he chose.