THE EVOLUTION OF PLANTS. 
21 
"had when young few at last remain. His concluding sentence is this : 
" As buds give rise by growth to fresh buds, and these, if vigorous, branch 
out and overtop on all sides many a feebler branch, so by generation, I 
believe, it has been with the great Tree of Life, which fills with its dead 
and broken branches the crust of the earth, and covers the surface with 
its ever-branching and beautiful ramifications." 
I have now shown, so far as I believe to be necessary, what evolution 
in nature is ; but before I deal with garden evolution, and briefly strike 
s, parallel, I should like to justify my reference to hybrids in a state of 
nature. Do I say that species in a state of nature sometimes originate 
as natural hybrids ? Such indeed is my belief; and, if there were no kind 
of evidence to be had outside the garden, I believe that this conclusion is 
inevitable. We know as a positive fact that hybrids are found wild in 
nature. We know also that in gardens we have hybrids of two extreme 
types. We have sterile hybrids, so dear to the heart of those who believe 
in a strict code of morality for plants, of their own prescription, and we 
have fertile hybrids which behave perfectly, as I shall show, like any so- 
called pure species. We have, in fact, besides these extremes every inter- 
mediate kind of hybrid, and it does appear to me most illogical to 
suppose that hybrids in nature can only be of one kind, and that of the 
sterile kind. It is certain, to my mind, that Nature, with all her oppor- 
tunities and length of days, must have a large number of species, which, 
if their genesis could be known, would prove to be hybrid. What would 
happen, supposing that we did casually meet with a wild hybrid, apart 
from obvious parents, which behaves like the so-called pure species ? 
Simply that we should regard it as an ordinary species, and in this way, 
I suspect, hybrid origin is often overlooked. Hybrids are often stopped 
by sterility, no doubt ; but it appears likely that in some cases partial 
sterility may give way to complete fertility.* Let me show by two instances, 
from Kerner, how hybrids can and do arise, originating species which 
take a place not filled by either parent. Salvia sylvestris is a hybrid 
between S. nemorosa and S. pratensis, growing in dry meadows all over 
the low country to the south of Vienna. The elevations are composed of 
boulders and clay, and wherever the latter is present in great quantities, 
••especially on the gentle slopes, Salvia nemorosa composes an important 
item of the vegetation. In the hollows, filled with dark moist earth, 
Salvia pratensis grows luxuriantly. The two kinds of habitat pass into 
each other, and so the parent species are able to meet. They have 
produced a hybrid, the Salvia nemorosa ; and it so happens that it prefers 
a habitat not liked by either parent. By experiment the hybrid has been 
proved fertile in a proportion of more than 60 per cent. The other 
instance I shall give is NujjJiar intermedium, a hybrid between N. luteum 
and N. pumilum. Its distribution extends further to the north than is 
the case with either parent. Neither can follow it. N. luteum drops 
-out first, because its fruits are the latest to ripen. N. intermedium 
ripens its fruit earlier than N. immilum, and can thus extend further 
north than that species. It is interesting that the hybrid produced 
artificially in the Botanic Gardens at Konigsberg is much less fertile than 
* Darwin instances hybrids between JEgilops and Wheat [Animah and Plants 
,U7ider Domestication, vol. ii. p. 88), 
