Darwin- Wallace Celebration . 103 



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 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 extermination ; it is evident that, of all the 

 individuals composing the species, those forming 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 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. 

 But this new, improved, and populous race might itself, in 

 course of time, give rise to new varieties, exhibiting several 

 diverging modifications of form, any of which, tending to 

 increase the facilities for preserving existence, must, by the 

 same general law, in their turn become predominant. Her*, 

 then, we have progression and continued divergence deduced 

 from the general laws which regulate the existence of animals 



