The crossed plants continued to the tenth generation to vary in the same manner as before, but to a much less degree, owing, probably, to their having become more or less closely inter-related. We must therefore attribute the extraordinary uniformity of colour in the flowers on the plants of the seventh and succeeding self-fertilised generations, to inheritance not having been interfered with by crosses during several preceding generations, in combination with the conditions of life having been very uniform.

A plant appeared in the sixth self-fertilised generation, named the Hero, which exceeded by a little in height its crossed antagonist, and which transmitted its powers of growth and increased self-fertility to its children and grandchildren. A cross between the children of Hero did not give to the grandchildren any advantage over the self-fertilised grandchildren raised from the self-fertilised children. And as far as my observations can be trusted, which were made on very unhealthy plants, the great-grandchildren raised from intercrossing the grandchildren had no advantage over the seedlings from the grandchildren the product of continued self-fertilisation; and what is far more remarkable, the great-grandchildren raised by crossing the grandchildren with a fresh stock, had no advantage over either the intercrossed or self-fertilised great-grandchildren. It thus appears that Hero and its descendants differed in constitution in an extraordinary manner from ordinary plants of the present species.

Although the plants raised during ten successive generations from crosses between distinct yet inter-related plants almost invariably exceeded in height, constitutional vigour, and fertility their self-fertilised opponents, it has been proved that seedlings raised by intercrossing flowers on the same plant are by no means superior, on the contrary are somewhat inferior in height and weight, to seedlings raised from flowers fertilised with their own pollen. This is a remarkable fact, which seems to indicate that self-fertilisation is in some manner more advantageous than crossing, unless the cross brings with it, as is generally the case, some decided and preponderant advantage; but to this subject I shall recur in a future chapter.

The benefits which so generally follow from a cross between two plants apparently depend on the two differing somewhat in constitution or character. This is shown by the seedlings from the intercrossed plants of the ninth generation, when crossed with pollen from a fresh stock, being as superior in height and almost as superior in fertility to the again intercrossed plants, as these latter were to seedlings from self-fertilised plants of the corresponding generation. We thus learn the important fact that the mere act of crossing two distinct plants, which are in some degree inter-related and which have been long subjected to nearly the same conditions, does little good as compared with that from a cross between plants belonging to different stocks or families, and which have been subjected to somewhat different conditions. We may attribute the good derived from the crossing of the intercrossed plants during the ten successive generations to their still differing somewhat in constitution or character, as was indeed proved by their flowers still differing somewhat in colour. But the several conclusions which may be deduced from the experiments on Ipomoea will be more fully considered in the final chapters, after all my other observations have been given.



Mimulus luteus; height, vigour, and fertility of the crossed and self-fertilised plants of the first four generations. Appearance of a new, tall, and highly self-fertile variety. Offspring from a cross between self-fertilised plants. Effects of a cross with a fresh stock. Effects of crossing flowers on the same plant. Summary on Mimulus luteus. Digitalis purpurea, superiority of the crossed plants. Effects of crossing flowers on the same plant. Calceolaria. Linaria vulgaris. Verbascum thapsus. Vandellia nummularifolia. Cleistogene flowers. Gesneria pendulina. Salvia coccinea. Origanum vulgare, great increase of the crossed plants by stolons. Thunbergia alata.

Charles Darwin

All Pages of This Book