58 MESSES. C. DAEWIN AND A. WALLACE ON THE 



country which they inhabit. It is also evident that most changes 

 would affect, either favourably or adversely, the powers of pro- 

 longing 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 diminu- 

 tion 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 therefore 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 numerical superiority. Now, let some alteration 

 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 exter- 

 mination ; it is evident that, of all the individuals composing the 

 species, those forming the least numerous and most feebly organ- 

 ized 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 circumstances would rapidly 

 increase in numbers and occupy the place of the extinct species 

 and variety. 



The variety would now have replaced the species, of which it 

 would be a more perfectly developed and more highly organized 

 form. It would be in all respects better adapted to secure its 

 safety, and to prolong its individual existence and that of the race. 

 Such a variety could not return to the original form ; for that form 

 is an inferior one, and could never compete with it for existence. 

 Granted, therefore, a " tendency" to reproduce the original type 

 of the species, still the variety must ever remain preponderant in 

 numbers, and under adverse physical conditions again alone survive. 



