ii ON THE TENDENCY OF VARIETIES, ETC. 27 



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 accurately tracing 

 the effects to their causes ; but could we become perfectly 

 acquainted with the organisation 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 pro- 

 portionate 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 kept 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 

 organisation and resulting habits, which, rendering it more difficult 

 to procure a regular supply of food and to provide for their personal 

 safety l 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 application. 



Useful Variations mil tend to Increase ; useless or hurtful 

 Variations to Diminish 



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 country which they could in- 



1 ' ' And that of their offspring " should have been added. But it must 

 be remembered that the writer had no opportunity of correcting the proofs of 

 this paper. 



