210 A NATURALIST ON DESERT ISLANDS. 



difierent conditions calling for different modifications. 

 This stimulus, which had been sufficient before, in the 

 presence of a plastic plasm, to conjure up new species, is 

 non-existent. The conditions of life, as regards these 

 birds (and the same applies to other species of 

 gannets which we need not mention) throughout all 

 the tropical seas, seem apparently to be remarkably 

 similar. 



To use a geological term, the species-producing forces 

 seem to have arrived at a " dead level of erosion," and, 

 just as in the cutting out of valleys by rivers, there comes 

 a time when the forces represented by the eroding waters 

 and the resistance offered by the rocks are nicely 

 balanced ; so in these three widely distributed gannets, 

 we seem to have arrived at a point where the tendency of 

 the plastic somatic plasm to vary, and the tendency of 

 the germ plasm to go on producing on the same lines, 

 are evenly matched. 



Isolation, of itself alone, does not seem capable of 

 producing fresh varieties any more than segregation. 

 Natural selection is only the final arbiter in determining 

 what variations shall survive, after they have been 

 produced by the influence of external conditions. If 

 the external conditions are the same all the world over, 

 natural selection cannot come into action. 



How long, then, are these three species going to con- 

 tinue the even tenour of their way ? How long is Nature 

 going to have, as regards these particular birds, three 

 strings to her bow ? In other words, when is the elimina- 

 tive process of the survival of the fittest going to make 

 itself evident ? 



At the present time, so thoroughly has natural selection 

 done her work in the past, in the case of each separate 

 species under mention, producing in each case such a 

 fixity of type and such a perfect adaptation to conditions, 

 different as these adaptations are, that a state of stable 

 equilibrium seems to have been brought about. It may 

 be, however, that we are in too great a hurry, and that, 



