NATURAL SELECTION 



reason why woodpeckers are scarce with us, while in the 

 tropics they are among the most abundant of solitary birds. 

 Thus the house sparrow is more abundant than the redbreast, 

 because its food is more constant and plentiful, seeds of 

 grasses being preserved during the winter, and our farm-yards 

 and stubble-fields furnishing an almost inexhaustible supply. 

 Why, as a general rule, are aquatic, and especially sea-birds, 

 very numerous in individuals 1 Not because they are more 

 prolific than others, generally the contrary ; but because their 

 food never fails, the sea-shores and river-banks daily swarm- 

 ing with a fresh supply of small mollusca and Crustacea. 

 Exactly the same laws will apply to mammals. Wild cats 

 are prolific and have few enemies ; why then are they never 

 as abundant as rabbits ? The only intelligible answer is, that 

 their supply of food is more precarious. It appears evident, 

 therefore, that so long as a country remains physically un- 

 changed, the numbers of its animal population cannot 

 materially increase. If one species does so, some others 

 requiring the same kind of food must diminish in proportion. 

 The numbers that die annually must be immense ; and as the 

 individual existence of each animal depends upon itself, those 

 that die must be the weakest the very young, the aged, and 

 the diseased while those that prolong their existence can 

 only be the most perfect in health and vigour those who are 

 best able to obtain food regularly, and avoid their numerous 

 enemies. It is, as we commenced by remarking, " a struggle 

 for existence," in which the weakest and least perfectly 

 organised must always succumb. 



The Abundance or Rarity of a Species dependent upon its more or 



less perfect Adaptation to the Conditions of Existence 

 It seems evident that what takes place among the indi- 

 viduals of a species must also occur among the several allied 

 species of a group, viz., that those which are best adapted 

 to obtain a regular supply of food, and to defend themselves 

 against the attacks of their enemies and the vicissitudes of the 

 seasons, must necessarily obtain and preserve a superiority in 

 population ; while those species which, from some defect of 

 power or organisation, are the least capable of counteracting 

 the vicissitudes of food-supply, etc., must diminish in numbers, 



