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.
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