166 THE OLDER HYBRIDISTS 
botanists regarded as good species, to be perfectly 
fertile together, he immediately regarded them as 
being only varieties of a single species. 
One curious point is worth quoting in this connec- 
tion. Five varieties of Nicotiana tabacum were found 
to be perfectly fertile with one another, but when 
crossed with Nicotiana glutinosa one of them was 
found to be distinctly less sterile than the rest. 
Another interesting point observed by Kolreuter 
was the fact that hybrid plants often exceed their 
parents in luxuriance of growth. Upon this fact, as 
we shall see later on, Knight and afterwards Darwin 
based theoretical conclusions of considerable impor- 
tance in connection with the problem of sex. 
Thomas Andrew Knight, who was also a botanist of 
high reputation in other fields, was the earliest observer 
to lay stress upon the practical aspect of the study of 
hybrids, and he occupied himself to a considerable 
extent with the improvement of useful races of plants 
by cross-breeding. Breeders of animals had already 
made important improvements by the method of inter- 
crossing different races, and selecting the most notable 
types which made their appearance in consequence, 
when Knight bethought him of applying the same 
principles to the improvement of plants, and particu- 
larly of fruit-trees. 
Knight also carried out a series of experiments with 
domestic peas, the results of which were published in 
1779. These experiments have a particular interest 
from the historical point of view, since it was by dint 
of similar experiments upon the same kind of plants 
