TENDENCY OF Si?EOIES TO TOEM VAEIETIES. 57 



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, &c., 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 wiU generally prevent us from accurately tracing the 

 effects 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 cotmtry is generally stationary, 

 heing kept down hy a periodical deficiency of food, and other checks \ 

 and, 2nd, that the comparative abundance or scarcity of the indi- 

 viduals of the several species is entirely due to their organization and 

 resulting habits, which, rendering it more difficult to procure a 

 regular swpply of food and to provide for their personal safety in 

 some cases than in others, can only be balanced by a difference in 

 the population which have to exist in a given area — we shall be in 

 a condition to proceed to the consideration of varieties, to which 

 the preceding remarks have a direct and very important appli- 

 cation. 



Most or perhaps all the variations from the typical form of a 

 species must have some definite effect, however slight, on the 

 habits or capacities of the individuals. Even a change of colour 

 might, by rendering them more or less distinguishable, affect their 

 safety ; a greater or less development of hair might modify their 

 habits. More important changes, such as an increase in the power 

 or dimensions of the limbs or any of the external organs, would 

 more or less affect their mode of procuring food or the range of 



