120 
DARWINISM 
CHAP. 
and recognition by members of the same variety or incipient 
species. It lias also been observed that each differently 
coloured variety of wild animals, or of domesticated animals 
which have run wild, keep together, and refuse to pair with 
individuals of the other colours ; and this must of itself act to 
keep the races separate as completely as physical isolation. 
On the Advance of Organisation by Natural Selection. 
As natural selection acts solely by the preservation of use¬ 
ful variations, or those which are beneficial to the organism 
under the conditions to which it is exposed, the result must 
necessarily be that each species or group tends to become more 
and more improved in relation to its conditions. Hence we 
should expect that the larger groups in each class of animals 
and plants—those which have persisted and have been abundant 
throughout geological ages—would, almost necessarily, have 
arrived at a high degree of organisation, both physical and 
mental. Illustrations of this are to be seen everywhere. 
Among mammalia we have the carnivora, which from Eocene 
times have been becoming more and more specialised, till they 
have culminated in the cat and dog tribes, which have reached 
a degree of perfection both in structure and intelligence fully 
equal to that of any other animals. In another line of 
development, the herbivora have been specialised for living 
solely on vegetable food till they have culminated in the sheep, 
the cattle, the deer, and the antelopes. The horse tribe, 
commencing with an early four-toed ancestor in the Eocene 
age, has increased in size and in perfect adaptation of feet and 
teeth to a life on open plains, and has reached its highest per¬ 
fection in the horse, the ass, and the zebra. In birds, also, we 
see an advance from the imperfect tooth-billed and reptile¬ 
tailed birds of the secondary epoch, to the wonderfully 
developed falcons, crows, and swallows of our time. So, the 
ferns, lycopods, conifers, and monocotyledons of the palteozoic 
and mesozoic rocks, have developed into the marvellous wealth 
of forms of the higher dicotyledons that now adorn the earth. 
But this remarkable advance in the higher and larger groups 
does not imply any universal law of progress in organisation, 
because we have at the same time numerous examples (as has 
been already pointed out) of the persistence of lowly organised 
