INFLUENCE OF ISOLATION ON THE FORMATION OF SPECIES 289 



forms show themselves especially in the male sex, as colour varia- 

 tions of certain parts of the plumage, we must take account of 

 sexual selection, which, though with its basis in germinal selection, 

 has in many islands followed a patli of its own. Sexual selection 

 operates especially in the case of sporadically occurring characters 

 which are in any way conspicuous. But it is just such variations as 

 these that are called into existence by germinal selection, whenever 

 it is allowed to continue its course undisturbed through a long series 

 of generations. Characters of this kind, such, for instance, as feathers 

 of abnormal structure or colour in a bird, or new colour spots in 

 a butterfly, make their appearance when a group of determinants has 

 been able to go on varying in the same direction for a long time un- 

 impeded, that is, without being eliminated as injurious by natural 

 selection or obliterated by crossing. This is very likely to happen in 

 the case of an isolated area, and as soon as the conspicuous character 

 thus brought about makes its appearance, sexual selection takes control 

 of it, and ensures that all the individuals, that is, all the germ- 

 plasms which possess it, have the preference in reproduction. 



I believe, therefore, that a large number of the endemic species of 

 birds and butterflies in isolated regions result from amixia based upon 

 germinal selection, whose results have been emphasized by sexual 

 selection. Experience corroborates this, as far as I can see, for many 

 of the endemic species of birds in the Galapagos and other islands 

 differ from one another solely or mainly in their colouring, and in 

 many it is especially the males which differ greatly. 



As to the humming-birds we may say, without going into details 

 regarding their sexual characters and their distribution, that the 

 many endemic species which inhabit the Alpine regions of isolated 

 South American volcanic mountains differ from one another chiefly in 

 the males and in the secondary sexual characters of these. The 

 family of humming-birds is characteristically Neotropical, that is, it 

 has its centre in the Tropics of the New World, and by far the greater 

 number of humming-bird species— there are about a hundred and fifty 

 — occur there only, while a few occur as migrants north of the Tropical 

 zone, and visit the United States as far north as Washington and 

 New York. We know that many of the most beautiful species have 

 quite a small area of distribution, that many are restricted to a single 

 volcanic mountain, living in the forests which clothe its sides. Tliese 

 species are isolated there, for they do not migrate ; apparently they 

 cannot endure the climate of the plains, but remain always in their 

 mountain forests. Without doubt they originated there, chiefly, I am 

 inclined to think, through the variation of the males due to sexual 

 II. u 



