ed 
—tts, 
136 CANNON: STUDIES IN PLANT HyBrRIDs 
became a question of great importance, since by this means it was 
hoped that a distinction between varieties and species might be had. 
Early in the nineteenth century the question was taken up by 
English and later by French botanists and since the facts were not 
well known the bearing of them on the question under discussion 
was also much in dispute. In England, Herbert * and Knight + 
discussed the relative fertility of species- and variety-hybrids, 
the latter author holding that hybrids from parents of different 
species are always sterile, but on the other hand that the varietal 
hybrids are always fertile. From the results of his experiments 
and study Knight concluded also that fruitful hybrids indicate 
that the parents are not distinct species, but varieties of the 
same species. Herbert on the contrary found that hybrids between 
different species are not seldom fertile. As to the significance of 
this he does not appear to think the parents were of the same 
species, but rather (4. c. 16) that they ‘have branched out from 
one common stock since the creation of the world.” He further 
says: ‘If it be admitted, that diversity of species could have been 
produced by variations of soil, temperature, or humidity, it will be 
readily understood that such diversity might have been further 
multiplied by hybrid intermixture, as the species were brought 
together by the natural progress of their diffusion.” 
It will not be necessary for the purposes of this paper to enter 
more into the details of this phase of the ‘“‘constancy of species” 
doctrine. The ground that Knight and Herbert threshed over in 
England was gone over again later (1862) in France by Naudin 
and Godron.{ Finally, Darwin § has pointed out that not only 
is the fertility or sterility of a hybrid no criterion of the relationship 
of the parents, but that the hybrid may even be more fruitful than 
either pure parent. 
Focke (/. c. 443) says that up to the time of Nageli (1865) the 
knowledge of hybrids was unconnected and fragmentary, and that 
this author united and made coherent the results of the earlier 
hybridizers. Especially he drew consistent conclusions from the 
(eieracailata aed 
* Herbert, On the Production of Hybrid Vegetables. ‘Trans. Hort. Soc. Lond. 4+ 
a. 1820, 
+ Knight, Observations on Hybrids. Trans. Hort. Soc. Lond. 4: 367- 1821- 
t Focke, Die Pflanzen-Mischlinge, 441. 1881. 
% Darwin, On the Origin of Species, Am. ed., 235. 1883. 
