V 
NATURAL SELECTION 
115 
are the lowest mammals—the echidna and ornithorhynclms 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 burrow¬ 
ing 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 Neiv Species hy 
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 condi¬ 
tions change, to however small an extent, some correspond¬ 
ing 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 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 u'ould, 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 
