340 
ON CROSSES AND 
has singularly the manners as well as the voice of a fox, but 
it is the parent of many families of puppies: and I feel satis- 
fied that the fox and the dog are of one origin, and suspect 
the wolf and jackall to be of the same; nor could I ever con- 
template the black line down the back of a dun pony with- 
out entertaining a suspicion that the horse, unknown in a 
wild state except where it has escaped from domesticity, may 
be a magnificent improvement of the wild ass in the very 
earliest age of the world : bearing in mind, that both in the 
animal and vegetable creation, the diversities arising from 
inscrutable causes in the wild races of the forest, are of a more 
unalterable character than those which spring up under the 
care and cultivation of man. With respect to animals in 
their wild state, their union with their own species seems to 
be mainly guided by voice and smell, and in domesticity 
that instinctive preference is evidently much weaker, and 
the will to keep themselves distinct is therefore lessened. 
The various species of greenish wrens are so similar ,in plu- 
mage, that it requires nice examination to distinguish them, 
yet they have different notes, manners, and habits of build- 
ing their nests, even when in the same locality ; but we have 
no certainty that if their predilection for the voice and smell 
of their own race was weakened, they would not be capable 
of producing a fertile cross; and we draw our conclusions 
from a few instances of domestic mules between species 
which happen to be widely removed from each other, as the 
pheasant and fowl, the goldfinch and canary bird ; wdiereas 
we should apply to this subject, concerning which our know- 
ledge is very limited, the consideration of the fact ascer- 
tained concerning vegetables, which have no will to interfere 
with our experiments, that some crosses are sterile and some 
quite fertile, without any apparent reason, except the greater 
or less approximation of constitution in the parents : and that 
the cross-bred plant, which has seemed for a long course of 
years to be absolutely sterile, becomes under some circum- 
stances productive. 
In accordance with the principle above stated, when it 
was shown that the botanic subdivisions of Rhododendron, 
Azalea, and Rhodora, comprehended plants which were 
capable of intermixing, I asserted that the botanist must 
reconsider and alter his subdivisions, and acknowledge that, 
notwithstanding their peculiarities, they constituted but one 
genus or kind. Conformably with this suggestion, Mr. 
