CONTENTS.

INTRODUCTION.

The Problems of Heredity and their Solution, pp. 1–39.

Preliminary statement of Mendel’s principles, 8. Relation of Mendel’s discovery to the law of Ancestral Heredity, 19. Heterozygote and Homozygote, 23. New conceptions necessitated by Mendel’s discovery, 26. Simple alternative characters, or allelomorphs, 27. Compound allelomorphs and their components, 29. Analytical Variations, 29. Relation of Mendel’s principle to continuous variation, 32. Dominance, 32. Non-Mendelian phenomena, 33. False hybrids of Millardet, 34. Brief historical notice, 36.

MENDEL’S EXPERIMENTS IN PLANT HYBRIDISATION, pp. 40–95.

Introductory Remarks, 40. Selection of Experimental Plants, 42. Division and Arrangement of Experiments, 44. Characters selected, 45. Number of first crosses, 47. Possible sources of error, 47. Forms of the Hybrids, 49. Dominant and recessive, 49.

First generation bred from the Hybrids, 51. Numbers of each form in offspring, 52. Second generation bred from the Hybrids, 55. Subsequent generations bred from the Hybrids, 57.

Offspring of Hybrids in which several differentiating characters are associated, 59. The reproductive cells of the Hybrids, 66. Statement of Mendel’s essential deductions, 67. Experiments to determine constitution of germ-cells, 68. Statement of purity of germ-cells, 72.

Experiments with Phaseolus, 76. Compound characters, 80. Concluding Remarks, 84.

MENDEL’S EXPERIMENTS WITH HIERACIUM, 96–103.

A DEFENCE OF MENDEL’S PRINCIPLES OF HEREDITY, 104–208.

Introductory, 104.

I.The Mendelian Principle of Purity of Germ-cells and the Laws of Heredity based on Ancestry, 108.

II.Mendel and the critic’s version of him.

The Law of Dominance, 117.

III.The facts in regard to Dominance of Characters in Peas, 119.

The normal characters: colours of cotyledons and seed-coats, 120. Shape, 122. Stability and variability, 124. Results of crossing in regard to seed-characters: normal and exceptional, 129. Analysis of exceptions, 132. The “mule” or heterozygote, 133.

IV.Professor Weldon’s collection of “Other evidence concerning Dominance in Peas.”

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.