to others. The most common
effect, therefore, will be that some species will increase and others
will diminish; and in cases where a species was already small in numbers
a further diminution might lead to extinction. This would afford room
for the increase of other species, and thus a considerable readjustment
of the proportions of the several species might take place. When,
however, the change was of a more important character, directly
affecting the existence of many species so as to render it difficult for
them to maintain themselves without some considerable change in
structure or habits, that change would, in some cases, be brought about
by variation and natural selection, and thus new varieties or new
species might be formed. We have to consider, then, which are the
species that would be most likely to be so modified, while others, not
becoming modified, would succumb to the changed conditions and become
extinct.

The most important condition of all is, undoubtedly, that variations
should occur of sufficient amount, of a sufficiently diverse character,
and in a large number of individuals, so as to afford ample materials
for natural selection to act upon; and this, we have seen, does occur in
most, if not in all, large, wide-ranging, and dominant species. From
some of these, therefore, the new species adapted to the changed
conditions would usually be derived; and this would especially be the
case when the change of conditions was rather rapid, and when a
correspondingly rapid modification could alone save some species from
extinction. But when the change was very gradual, then even less
abundant and less widely distributed species might become modified into
new forms, more especially if the extinction of many of the rarer
species left vacant places in the economy of nature.

_Probable Origin of the Dippers._

An excellent example of how a limited group of species has been able to
maintain itself by adaptation to one of these "vacant places" in nature,
is afforded by the curious little birds called dippers or water-ouzels,
forming the genus Cinclus and the family Cinclidae of naturalists. These
birds are something like small thrushes, with very short wings and tail,
and very dense plumage. They frequent, exclusively, mountain torrents in
the northern hemisphere, and obtain their food entirely in the water,
consisting, as it does, of water-beetles, caddis-worms and other
insect-larvae, as well as numerous sma