TENDENCY OF SPECIES TO FORM VARIETIES. 17 



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 application. 



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

 must have some definite effect, however slight, on the habits or capac- 

 ities 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 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 inhabit. It is 

 also evident that most changes would affect, either favourably or ad- 

 versely, the powers of prolonging existence. An antelope with shorter 

 or weaker legs must necessarily suffer more from the attacks of the 

 feline carnivora; the passenger pigeon with less powerful wings would 

 sooner or later be affected in its powers of procuring a regular supply 

 of food; and in both cases the result must necessarily be a diminution 

 of the population of the modified species. If, on the other hand, any 

 species should produce a variety having slightly increased powers of 

 preserving existence, that variety must inevitably in time acquire a 

 superiority in numbers. These results must follow as surely as old age, 

 intemperance, or scarcity of food produce an increased mortality. In 

 both cases there may be many individual exceptions ; but on the average 

 the rule will invariably be found to hold good. All varieties will there- 

 fore fall into two classes — those which under the same conditions would 

 never reach the population of the parent species, and those which would 

 in time obtain and keep a num.erical superiority. Now, let some alter- 

 ation of physical conditions occur in the district — a long period of 

 drought, a destruction of vegetation by locusts, the irruption of some 

 new carnivorous animal seeking 'pastures new* — any change in fact 

 tending to render existence more difficult to the species in question, 

 and tasking its utmost powers to avoid complete extermination; it 

 is evident that, of all the individuals composing the species, those form- 

 ing the least numerous and most feebly organized variety would suffer 

 first, and, were the pressure severe, must soon become extinct. The 

 same causes continuing in action, the parent species would next suffer, 

 would gradually diminish in numbers, and with a recurrence of similar 

 unfavourable conditions might also become extinct. The superior 

 variety would then alone remain, and on a return to favourable circum- 

 stances would rapidly increase in numbers and occupy the place of the 

 extinct species and variety. 



VOL. LX. — 2. 



