THE ORIGIN OF SPECIES 67 



If external conditions remain the same, the species will remain 

 fairly stable in its character ; all the weaklings will be weeded 

 out, and as time goes on an ever higher standard of fitness 

 will be found amongst the survivors. But if conditions change 

 for the whole species or for a part of it which has wandered 

 into a new environment then a different standard of excel- 

 lence will be required in the survivors, and a new race differing 

 in character from the parent race will slowly come into being 

 through the mating together of survivors possessing slightly 

 different qualities from those called for by the old conditions. 

 In this way as a species spreads it will slowly break up into 

 daughter species, distributed over different regions of the globe, 

 or distributed into different situations in the same region 

 of the globe. These daughter species in course of time will 

 come to differ from one another so much that they will be 

 assigned to different genera, and by a continuance of this 

 process the vast complexity of classes, orders, tribes, families, 

 genera, and species within the limits of a phylum will come 

 about. All the members of a phylum are descendants of a 

 single species, but Darwin hesitated to assert that such 

 different phyla as Arthropoda, Mollusca, and Vertebrata are 

 descendants of a common type of ancestor. He rather inclined 

 to the belief that if life apppeared on this globe by a process 

 which we know nothing about, it may have appeared simul- 

 taneously in several different forms whose descendants remain 

 as the different phyla to-day. It has been the task of suc- 

 ceeding zoologists to show that even phyla so distinct as 

 Arthropoda and Mollusca afford evidence, when closely ex- 

 amined, of having been derived at some inconceivably remote 

 epoch from common ancestors. But in order to complete 

 Darwin's task it was necessary that he should show that the 

 constitutional differences between species which express them- 

 selves in their inability to breed together are paralleled by 

 minor differences of the same kind between breeds. Thie 

 is a part of the work in which he was less successful than 

 he was in demonstrating the existence of natural selection. 

 But he showed that sterility can exist between different forms 

 of the same species. Thus among plants there is the familiar 

 example of the primrose, in which there are two forms of 

 flower, one with tall stamens and a short pistil, and the other 



