Systematic and Sc. vital Relationship. 595 
apply the principle in those numerous cases in which they 
fail. With regard to the former point, it should be noted 
that there are numerous natural hybrids which cannot as 
yet be made artificially, as for instance Ribes Gordonia- 
num ; in other words that many possible hybridizations 
do not succeed within the narrow limits of an experiment. 
The impossibility of a successful cross can therefore 
hardly ever be proved experimentally. With regard to 
the second point it suffices to cite the fact that in the 
great majority of genera no specific hybrids exist at all ; 
and that therefore here the delimitation of genera accor- 
ding to this principle, would fail entirely. 
We come now to the species. KOLREUTER expressed 
the view that crosses within the limits of these are fertile 
and give fertile offspring; whereas crosses between spe- 
cies would either show a lessened fertility or at least 
would produce infertile hybrids. GARTNER and most of 
the more recent investigators have subscribed to this 
view, except that they regard diminished fertility, rather 
than the actual absence of it, as the index of the bound- 
aries of the species. 1 
But even to these generalizations the exceptions are 
so numerous that unanimity in their application has not 
yet been reached. The parallel between sexual affinity 
and systematic relationship holds good in general, but 
fails only too often in particular cases. 2 NAUDIN re- 
garded these deviations from the rule as exceptions, 3 and 
ABBADO and several other investigators have claimed the 
determining cause of these exceptions in individual cases 
to be the task of hybridological researches. 4 
PARTNER, loc. cit., pp. 163-164, 578-579, etc. 
2 See MURBECK, Botaniska Notiser, 1901, p. 214. 
3 CH. NAUDIN, L'Hybridite dans les vegctaiix, 1869, P- 145- 
4 ABBADO, L'ibridisino net vegetali, 1898, p. 48. 
