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species would, under any change of circumstances, die, and, of course, 
the species would die with them. Now, it is not difficult to prove 
that this power of modification is possessed by plants and animals, 
I may instance the change which takes place in the wool of sheep, 
according as the animal is transferred from one climate to another — - 
the change in the size of the chest and lungs which is said to take 
place in the second generation of animals transported from ordinary 
elevations to the intensely rarified air of lofty mountains, or the 
alteration that is found in shells, whether fresh-water or marine, 
when transferred into brackish water. But for evidence of this I 
need not go beyond the examples given by Mr Darwin himself. I 
think that all the instances of variation mentioned by him may be 
referred to this principle of modification. To this principle, and as 
designed for a similar purpose, do I refer the phenomena of hybridi- 
zation. Putting aside a few exceptional cases, which may be explained 
on special grounds, I conceive that the well-known and undeniable 
general fact, that two distinct species may produce hybrid offspring, 
which hybrid offspring will be sterile either in the first or second 
generation, is strictly an instance of modification, allowed and in- 
tended for the preservation of the species. Conceive, for instance, 
a herd of deer, or any other animal, of which all the males have 
died off-— conceive it to be the last herd of that species on the face of 
the earth. Except for this power of hybridization, the species is 
extinct, although it yet lives. Its propagation is at an end. No 
young can replenish its numbers, and the species endures only until 
the last individual has died off. But with the power of having fer- 
tile intercourse with a distinct species, another chance is given for 
its preservation. A hybrid is born ; and if a male, it can have fer- 
tile offspring from the females of the original herd, and in a few 
generations all trace of the foreign blood will have been washed out. 
Such, I conceive, to be the uses of the principle of modification in its 
various phases, viz., the preservation of the species by the preser- 
vation and propagation of the individual. But the species may be 
lost in another way than by the death of its component individuals. 
It might, were there no check upon this power of modification, be 
lost by hybrids and modified individuals taking the place of species ; 
in fact, were the power of variation unlimited and uncontrolled, all 
species would be confounded, and there would be nothing but an 
indiscriminate mass of creatures running all into each other, as 
