HYBRIDISATION AND CROSS-BREEDING. 65 
also occurred when the cross-bred plants were bred with each other. 
Professor de Vries kindly sent seed of his glabrous form to Cambridge, 
- where Miss Saunders repeated the experiments with the same results. In 
all the cases of mixed progeny there is a sharp discontinuity. : 
The third case is that of Biscutella levigata. A full account of this 
important case was published by Miss Saunders in “ Proc. Roy. Soe.’’ 1897, 
vol. lxii. p. 11. Briefly the facts are as follows. The species is common 
as a hairy plant throughout a great part of the Alps. In a few localities 
a variety occurs having the swrfaces of the leaves quite devoid of hairs. 
(There are almost always some hairs on the margins and leaf-teeth.) 
When present, this smooth form occurs abundantly, mixed with the hairy 
type. Intermediates are of rare occurrence. If plants of the two kinds 
breed freely together, as in the natural state we must suppose they do, 
how is the sharp distinction in their respective characters maintained ? 
The result of artificial cross-breeding went to show that of the young 
seedlings of mixed parentage some were hairy, some smooth, and a good 
many intermediate. But as these seedlings grew, the hairy and the 
smooth retained their original characters, while the intermediate ones 
gradually became smooth. The transition was not effected by actual loss 
of hairs, but after the first few leaves of intermediate character the leaves 
subsequently produced were smooth. 
In all these three cases there is discontinuity, the intermediates between 
the varieties being absent or relatively scarce. Nevertheless, on examina- 
tion it is found that the discontinuity is not maintained in the same way 
in the different cases. The transmitting powers of the one variety in 
respect of the other are quite different in each case, and it must, I think, 
be admitted that we have here a fact of great physiological significance. 
In each of the three cases enumerated the two varieties are seen to stand 
towards each other in a different relation, and in each the mechanism of 
inheritance works differently. 
From facts like these we perceive how imperfect is the survey of the 
characteristics of species and varieties which can be obtained by the 
ordinary methods of anatomy and physiology. There can be no doubt 
that, tested by the method of breeding and by study of the transmitting 
powers, the relation of varieties and species would be shown in an entirely 
new light. Weare accustomed to speak of “ variability ’’ as though it 
were a single phenomenon common to all living things; and just as the 
older naturalists spoke of species in general as all fixed and comparable 
entities, so many of the present evolutionists speak of ‘“‘ varieties’: in 
general as all comparable. This is a mere slurring of the facts. Not 
only must variability in respect of different characters be a manifestation 
of distinct physical processes, but, as we have seen, variability, even in 
what appears to us to be the same character, may be a wholly different 
matter. 
Our business, then, is to test and examine these different kinds of 
variabilities according to their behaviour when the different varieties 
are crossed together. By this means we are enabled to investigate the. 
properties of organisms in a way that no other method provides. 
If I may be allowed to use a metaphor taken from chemical science, 
regarding species and varieties as substances, we may investigate their 
. 
