1 6 POPULAR SCIENCE MONTHLY. 



ply. Wh}^, as a general rule, are aquatic, and especially sea birds, very 

 numerous in individuals? Not because they are more prolific than 

 others, generally the contrary; but because their food never fails, 

 the sea-shores and river-banks daily swarming 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 unchanged, 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 vigor — those who are best able to obtain food 

 regularly, and avoid their numerous enemies. It is, as we commenced 

 by remarking, '& struggle for existence,' in which the weakest and least 

 perfectly organized must always succumb. 



Now it is clear that what takes place among the individuals 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 organization are the least capable of counteracting the 

 vicissitudes of food supply, etc., must diminish in numbers, and, in 

 extreme cases, become altogether extinct. Between these extremes the 

 species will present various degrees of capacity for ensuring the means 

 of preserving life; and it is thus we account for the abundance or 

 rarity of species. Our ignorance will generally prevent us from ac- 

 curately tracing the efl'ects to their causes ; but could we become perfectly 

 acquainted with the organization and habits of the various species of 

 animals, and could we measure the capacity of each for performing 

 the different acts necessary to its safety and existence under all the 

 varying circumstances by which it is surrounded, we might be able 

 even to calculate the proportionate abundance of individuals which is 

 the necessary result. 



If now we have succeeded in establishing these two points — 1st, 

 that the animal population of a country is generally stationary, being 

 I'ept down by a periodical deficiency of food, and other checks; and, 

 2d, that the comparative abundance or scarcity of the individuals of 

 the several species is entirely due to their orgo/nization and resulting 

 hahits, ivhich, rendering it more difficult to procure a regular supply of 



