nt is able to stand the change
and to perpetuate itself for many generations, it will have become so
changed that botanists will class it as a new species."[232]

"The same sort of process goes on in the animal kingdom, but animals are
modified more slowly than plants."[233]

The sterility of hybrids, to which Mr. Darwin devotes a great part of
the ninth chapter of his 'Origin of Species,'[234] is then touched
on--briefly, but sufficiently--as follows:--

"The idea that species were fixed and immutable involved the belief that
distinct species could not be fertile _inter se_. But unfortunately
observation has proved, and daily proves, that this supposition is
unfounded. Hybrids are very common among plants, and quite sufficiently
so among animals to show that the boundaries of these so-called
immutable species are not so well defined as has been supposed. Often,
indeed, there is no offspring between the individuals of what are called
distinct species, especially when they are widely different, and again,
the offspring when produced is generally sterile; but when there is less
difference between the parents, both the difficulty of breeding the
hybrid, and its sterility when produced, are found to disappear. In this
very power of crossing we see a source from which breeds, and ultimately
species, may arise."[235]

Mr. Darwin arrives at the same conclusion. He writes:--

"We must, therefore, either give up the belief of the universal
sterility of species when crossed, or we must look at this sterility in
animals, not as an indelible characteristic, but as one capable of being
removed by domestication.

"Finally, on considering all the ascertained facts on the intercrossing
of plants and animals, it may be concluded that some degree of
sterility, both in first crosses and in hybrids, is an exceedingly
general result, but that it cannot, under our present state of
knowledge, be considered as absolutely universal."[236]

Returning to Lamarck, we find him saying:--

"The limits, therefore, of so-called species are not so constant and
unvarying as is commonly supposed. Consider also the following. All
living forms upon the face of the globe have been brought forth in the
course of infinite time by the process of generation only. Nature has
directly created none but the lowest organisms; these she is still
producing every day, they being, as it were, the first sketches of life,
and produced by what is called spontaneous