THE EVOLUTION OF PLANTS. 
31 
not crossing we have forms of Maize quoted by Darwin, and Silene inflata 
rar. alpina with var. angustifolia and var. latifolia with var. littoralis 
quoted by Sachs. There is also the difficulty that plants will sometimes 
cross one way but not the other, or the fertility of one union is greater 
than the other. Sorauer remarks that while the pollen of Orchis Morio 
will not even produce a tube on the stigma of Orchis fusca, yet pollen of 
Orchis fusca actually fertilises the egg-cell of 0. Morio. Goertner says 
that Nicotiana paniculata is fertile with pollen of N. Langsdorfi, but 
that the reverse cross cannot be made. It is the same with species of 
Mirabilis, explicable perhaps in that case by the inability of the pollen 
tube of the one to traverse the long style of the other. A further 
difficulty may be mentioned in the fact that plants sometimes refuse to 
cross with their known allies and yet prove fertile with removed species. 
The late Anderson Henry, speaking oiBhododenclron jasminiflorum, said : 
While it rejected so many of its legitimate brethren of the Rhododen- 
dron tribe, pure and simple, I was somewhat surprised that it took kindly 
with my hybrid B. ciliatum crossed by B. Edgworthi.'' Also of the same 
species he wrote : "It again rejected its more natural allies and formed 
a union with the Indian Azalea." From these remarks it will be evident, 
I think, that we could not depend upon knowledge of the power of 
crossing or not crossing together as a guide to classification. 
Another interesting question, especially after the foregoing considera- 
tions, is whether we can make any distinction between hybrid and cross. 
Certainly we may for the sake of convenience call a plant a hybrid, when 
it is descended from two plants which we regard as specifically distinct, 
and we may call a plant a cross which is descended from plants which 
we look upon as varieties or forms of the same species. But species and 
varieties themselves are judgments, as Asa Gray used to say, and the 
terms apply rather to an abstract idea than to a concrete fact, and, 
apparently to be quite logical, we can get no further with the terms 
hybrid and cross. At any rate genetic affinity and sexual affinity are not 
concurrent, so that a cross from the one point of view might be a hybrid 
from the other. Focke used the one word Mischling for both hybrid and 
cross, and, apart from convenience, it would be better, probably, to use one 
term in a similar way. I introduce no new idea. Herbert long ago, and 
Naudin more recently, after many experiments, formed the opinion that 
it was impossible to draw the line between the cross and the hybrid. 
Any feature of the one may be a feature of the other. 
B. — Not of Hybeid Origin. 
From hybrids I pass to illustrations that are not of hybrid origin. To 
this class belong all the most anciently cultivated of domestic races. From 
the earliest period, when man began to cultivate, there must have been 
going on a process of selection, and to such an extent has the resulting 
evolution proceeded, that to-day we are unable to point out the originals 
of many of our most useful plants. New species have certainly been 
established, which must, at the very least, rank with species that have 
been evolved naturally. I do not think that a new genus has arisen, 
although it has been suggested that Zea, of which no wild species is 
known, has been evolved from Euchkena (Beana). Of this, in my opinion, 
