certain limit to normal variability, and that if changes take place they
may be expected _a priori_ to be marked and considerable ones, from the
facts of the inorganic world, and perhaps also of the lowest forms of the
organic world. It has also been seen that with regard to minute spontaneous
variations in races, there is a rapidly increasing difficulty in
intensifying them, in any one direction, by ever such careful breeding.
Moreover, it has appeared that different species show a tendency to
variability in special directions, and probably in different degrees, and
that at any rate Mr. Darwin himself concedes the existence of an internal
barrier to change when he credits the goose with "a singularly inflexible
organization;" also, that he admits the presence of an _internal_
proclivity to change when he speaks of "a whole organization seeming to
have become plastic, and tending to depart from the parental type."

We have seen also that a marked tendency to reversion does exist, inasmuch
as it sometimes takes place in a striking manner, as exemplified in the
white silk fowl in England, _in spite of_ careful selection in breeding.

Again, we have seen that a tendency exists in nature to eliminate hybrid
races, by whatever means that elimination is effected, while no similar
tendency bars the way to an indefinite blending of varieties. This has also
been enforced by statements as to the prepotency of certain pollen of
identical species, but of distinct races.

To all the preceding considerations have been added others derived from the
relations of species to past time. It has been contended that we have as
yet no evidence of minutely intermediate forms connecting uninterruptedly
together undoubtedly distinct species. That while even "horse ancestry"
fails to supply such a desideratum, in very strongly marked and exceptional
kinds (such as the Ichthyosauria, Chelonia, and Anoura), the absence of
links is very important and significant. For if every species, without{224}
exception, has arisen by minute modifications, it seems incredible that a
small percentage of such transitional forms should not have been preserved.
This, of course, is especially the case as regards the marine Ichthyosauria
and Plesiosauria, of which such numbers of remains have been discovered.

Sir William Thomson's great authority has been seen to oppose itself to
"Natural Selection," by limiting, on astronomical and physical grounds, the
duration