86 GENETICS AND EUGENICS 



fertilized with the pollen of A. But Ihe two opposite processes of fertili- 

 zation are not always equally easy to carry out. An extreme instance of 

 this circumstance was met with in the case of the genus Mirabilis. Mirab- 

 ilis jalapa was easily fertilized with pollen from M. longiflora. During 

 eight years Kolreuter made more than two hundred attempts to effect the 

 reverse cross, but without success. 



It was shown by Kolreuter that hybrids between different races or 

 varieties of the same species are usually much more fertile than hybrids 

 obtained by crossing distinct species. Indeed, he believed that varieties 

 of a single species were in all cases perfectly fertile together, whilst hybrids 

 between species always showed some degree of sterility. But in this case 

 Kolreuter based his definition of a species upon the very point at issue, 

 and when he found forms, which other 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 noting in this connection. 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 importance in connection with the 

 problem of sex. 



To pick out the salient features of the foregoing account 

 we may notice: 



1. That Kolreuter established the occurrence of sexual 

 reproduction in plants by showing that hybrid offspring in- 

 herit equally from the pollen plant and the seed plant. 



2. He showed that hybrids are commonly intermediate 

 between their parents in nearly all characters observed, such 

 for example as size and shape of parts. 



3. Many hybrids are partially or wholly sterile, especially 

 when the parents are very dissimilar (belong to widely dis- 

 tinct species). Such hybrids often exceed either parent 

 species in size and vigor of growth. 



4. Kolreuter did not observe the regular splitting of hy- 

 brids which Mendel and De Vries record, but some of his 

 successors did, particularly Thomas Knight (1799) ^ and John 

 Goss (1822) ^ in England who were engaged in the crossing of 

 garden peas with a view to producing more vigorous and 



^ For a fuller account of the work of these early plant hybridizers, see Lock. 



