XI 
THE SPECIAL COLOURS OF PLANTS 
327 
rigid selection or some change of conditions. Under nature, 
as in the case of the Porto Santo rabbits, the rapid increase of 
these animals would in a very few years stock the island with 
a full population, and thereafter natural selection would act 
powerfully in the preservation only of the healthiest and the 
most fertile, and under these conditions no deterioration 
would occur. Among the aristocracy there has been a 
constant selection of beauty, which is generally synonymous 
with health, while any constitutional infertility has led to the 
extinction of the family. With domestic animals the selec¬ 
tion practised is usually neither severe enough nor of the 
right kind. There is no natural struggle for existence, but 
certain points of form and colour characteristic of the breed 
are considered essential, and thus the most vigorous or the 
most fertile are not always those which are selected to 
continue the stock. In nature, too, the species always extends 
over a larger area and consists of much greater numbers, and 
thus a difference of constitution soon arises in different parts 
of the area, which is wanting in the limited numbers of pure 
bred domestic animals. From a consideration of these varied 
facts we conclude that an occasional disturbance of the organic 
equilibrium is what is essential to keep up the vigour and 
fertility of any organism, and that this disturbance may be 
equally well produced either by a cross between individuals 
of somewhat different constitutions, or by occasional slight 
changes in the conditions of life. Now plants which have 
great powers of dispersal enjoy a constant change of con¬ 
ditions, and can, therefore, exist permanently, or at all events, 
for very long periods, without intercrossing; while those 
which have limited powers of dispersal, and are restricted to 
a comparatively small and uniform area, need an occasional 
cross to keep up their fertility and general vigour. We 
should, therefore, expect that those groups of plants which are 
adapted both for cross- and self-fertilisation, which have showy 
flowers and possess great powers of seed-dispersal, would be 
the most abundant and most widely distributed; and this we 
find to be the case, the Composite possessing all these charac¬ 
teristics in the highest degree, and being the most generally 
abundant group of plants with conspicuous flowers in all parts 
of the world. 
