hanges, and
diversification of form and structure, in accordance with changed
conditions, their essential nature has probably remained the same from
the very dawn of life on the earth. The low aquatic diatomaceae and
confervae, together with the lowest fungi and lichens, occupy a similar
position in the vegetable kingdom, filling places in nature which would
be left vacant if only highly organised plants existed. There is,
therefore, no motive power to destroy or seriously to modify them; and
they have thus probably persisted, under slightly varying forms, through
all geological time.

_Extinction of Lower Types among the Higher Animals._

So soon; however, as we approach the higher and more fully developed
groups, we see indications of the often repeated extinction of lower by
higher forms. This is shown by the great gaps that separate the
mammalia, birds, reptiles, and fishes from each other; while the lowest
forms of each are always few in number and confined to limited areas.
Such are the lowest mammals--the echidna and ornithorhynchus of
Australia; the lowest birds--the apteryx of New Zealand and the
cassowaries of the New Guinea region; while the lowest fish--the
amphioxus or lancelet, is completely isolated, and has apparently
survived only by its habit of burrowing in the sand. The great
distinctness of the carnivora, ruminants, rodents, whales, bats, and
other orders of mammalia; of the accipitres, pigeons, and parrots, among
birds; and of the beetles, bees, flies, and moths, among insects, all
indicate an enormous amount of extinction among the comparatively low
forms by which, on any theory of evolution, these higher and more
specialised groups must have been preceded.

_Circumstances favourable to the Origin of New Species by Natural
Selection._

We have already seen that, when there is no change in the physical or
organic conditions of a country, the effect of natural selection is to
keep all the species inhabiting it in a state of perfect health and full
development, and to preserve the balance that already exists between the
different groups of organisms. But, whenever the physical or organic
conditions change, to however small an extent, some corresponding change
will be produced in the flora and fauna, since, considering the severe
struggle for existence and the complex relations of the various
organisms, it is hardly possible that the change should not be
beneficial to some species and hurtful