The Botanical Work of Darwin. xiii 
The mass of information gathered in the last-named way 
and by reading is given in his Variation of Animals and Plants 
under Domestication, which, like the Descent of Man and the 
Expression of the Emotions, were amplifications of parts of 
the Origin, in which he gave in full the evidence referred to in 
that so-called ‘ Abstract/ In the Variation of Animals and 
Plants he was also able to include more of his own researches, 
partly because, as already explained, it was published in 1868, 
six years after the Orchid book, and partly because it was 
a more appropriate place for minute observations. Here, for 
instance, are given the remarkable facts on the similarity, or 
rather identity, of the seeds of various kinds of Brassica, in 
which the leaves or other vegetative parts have by man’s 
selective power come to differ widely 1 . 
Among the subjects especially interesting to the botanist 
as distinguished from the horticulturist may be mentioned 
the contents of the well-known chapter xi of Vol. I, £ on 
bud variation, and on certain anomalous modes of reproduction 
and variation.’ In Vol. II, again, there are well-known 
chapters or paragraphs on crossing, on Knight’s Law 2 , on the 
sterility of cultivated varieties, on the good effect of crossing 
and on self-sterility, where his own work and his corre- 
spondence with Fritz Muller are prominent ; on the stimulating 
effect of changed conditions, and on the complex problem of 
sterility due to changed conditions ; on the difference in 
fertility between varieties and species when crossed, and on 
hybridism ; on the direct action of conditions. 
My father’s correspondence with Fritz Muller was, in its 
bearing on his work, second in importance only to that with 
Hooker. He had for Muller a stronger personal regard than 
that which bound him to his other unseen friends. Muller’s 
letters were vividly interesting, with their constant stream of 
new observation on many biological subjects. Moreover, 
1 Animals and Plants, ed. 1, Vol. i, p. 323. The collections of seeds made for 
this purpose are now in the Botanical Museum at Cambridge. 
3 I have elsewhere discussed Knight’s Law in relation to Darwin’s work ; see 
Nature , 1898. 
