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. AVith 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 \agorous 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 Avhat 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 indiAdduals 

 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 haA^e 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 Compositae possessing all these charac- 

 teristics in the highest degree, and being the most generally 

 abundant group of plants mth conspicuous flowers in all parts 

 of the world. 



