KNIGHT 155 



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 



