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.

CHAPTER III.

SCROPHULARIACEAE, GESNERIACEAE, LABIATAE, ETC.

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.

In the family of the Scrophulariaceae I experimented on species in the six following genera:  Mimulus, Digitalis, Calceolaria, Linaria, Verbascum, and Vandellia.

[3/2.  Scrophulariaceae.—­Mimulus luteus.

The plants which I raised from purchased seed varied greatly in the colour of their flowers, so that hardly two individuals were quite alike; the corolla being of all shades of yellow, with the most diversified blotches of purple, crimson, orange, and coppery brown.  But these plants differed in no other respect. (3/1.  I sent several specimens with variously coloured flowers to Kew, and Dr. Hooker informs me that they all consisted of Mimulus luteus.  The flowers with much red have been named by horticulturists as var.  Youngiana.) The flowers are evidently well adapted for fertilisation