STABILITY OF SPECIES. 209 



Now it is a certain fact that, if we could only go far 

 enough back, we should find that these three gannets, 

 so distinct to-day, had originally a comnxon ancestor. 

 In the course of the ages, however, by reason of the vicissi- 

 tudes of change and climate which the earth suffered, 

 we must presume that this common ancestral stock was 

 exposed to differences of surroundings and conditions, 

 which, aided by the influence of natural selection, eventu- 

 ally produced changes and modifications in the common 

 type ; so that in process of time the three species under 

 consideration were evolved. These differences of environ- 

 ment have now apparently ceased to exist ; and having 

 ceased, how is it that each species still remains distinct, 

 and continues to travel along the same lines down which 

 it was originally set going ? Why do they not tend to 

 retrograde, to revert to the original type, or to fuse into 

 one common species combining the good points of all 

 three ? We have everything present which is ordinarily 

 supposed to be conducive to the formation of new forms, 

 or the disappearance of existing ones — an inherently 

 unstable plasm, isolation, segregation and natural selection ; 

 and lastly we might add that eliminative force known 

 as the survival of the fittest. Yet under the same identical 

 conditions, the red-footed gannet still continues to pro- 

 duce its own peculiar colouring and plumage and to build 

 in trees, while the booby goes on producing its brown 

 coat, and lays its eggs on the grouind, and so on. 



It is perfectly certain, that in times past it must have 

 been a serviceable thing for the red-footed gannet to have 

 these distinctive peculiarities, and likewise useful for the 

 booby to have such a totally different plumage. Natural 

 selection had seen carefully to that, and had picked 

 and chosen among the countless variations, which had 

 been produced by an unstable protoplasm, liable to vary 

 in the presence of predisposing internal and external 

 conditions. 



But what is natural selection doing now ? There is 

 no question now (at least as far as we can appreciate) of 



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