odification which
has yet been advanced. It is one remarkable peculiarity of Mr. Darwin's
hypothesis that it involves no necessary progression or incessant
modification, and that it is perfectly consistent with the persistence
for any length of time of a given primitive stock, contemporaneously
with its modifications. To return to the case of the domestic breeds
of pigeons, for example; you have the Dove-cot pigeon, which closely
resembles the Rock pigeon, from which they all started, existing at the
same time with the others. And if species are developed in the same way
in nature, a primitive stock and its modifications may, occasionally,
all find the conditions fitted for their existence; and though they come
into competition, to a certain extent, with one another, the derivative
species may not necessarily extirpate the primitive one, or 'vice
versa'.

Now palaeontology shows us many facts which are perfectly harmonious
with these observed effects of the process by which Mr. Darwin supposes
species to have originated, but which appear to me to be totally
inconsistent with any other hypothesis which has been proposed. There
are some groups of animals and plants, in the fossil world, which have
been said to belong to "persistent types," because they have persisted,
with very little change indeed, through a very great range of time,
while everything about them has changed largely. There are families of
fishes whose type of construction has persisted all the way from the
carboniferous rock right up to the cretaceous; and others which have
lasted through almost the whole range of the secondary rocks, and from
the lias to the older tertiaries. It is something stupendous this--to
consider a genus lasting without essential modifications through all
this enormous lapse of time while almost everything else was changed and
modified.

Thus I have no doubt that Mr. Darwin's hypothesis will be found
competent to explain the majority of the phenomena exhibited by species
in nature; but in an earlier lecture I spoke cautiously with respect to
its power of explaining all the physiological peculiarities of species.

There is, in fact, one set of these peculiarities which the theory of
selective modification, as it stands at present, is not wholly competent
to explain, and that is the group of phenomena which I mentioned to you
under the name of Hybridism, and which I explained to consist in the
sterility of the offspring of certain