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.