TROPICAL NATURE 



variation, and with those other laws which I have endeavoured 

 to show regulate the development of colour. 1 



The amount of variation which is likely to occur in a 

 species will be greatly influenced by two factors the occur- 

 rence of a change in the physical conditions, and the average 

 abundance or scarcity of the individuals composing the species. 

 When from these or other causes variation occurs, it may 

 become fixed as a variety or a race, or may go on increasing 

 to a certain extent, either from a tendency to vary along 

 certain special lines induced by local or physiological causes, 

 or by the continued survival and propagation of all such 

 varieties as are beneficial to the race. After a certain time 

 a balance will be arrived at, either by the limits of useful 

 variation in this one direction having been reached, or by the 

 species becoming harmoniously adapted to all the surrounding 

 conditions : and without some change in these conditions the 

 specific form may then remain unaltered for a very long time ; 

 whence arises the common impression of the fixity of species. 

 Now in a country like Chili, forming part of a great continent 

 very well stocked with all forms of organic life, the majority 

 of the species would be in a state of stable equilibrium ; the 

 most favourable variations would have been long ago selected ; 

 and the numbers of individuals in each species would be 

 tolerably conBtant, being limited by the numerous other forms 

 whose food and habits were similar, or which in any way 

 impinged upon its sphere of existence. We may, therefore, 

 assume that the Chilian humming-bird which migrated to 

 Juan Fernandez was a stable form, hardly if at all different 

 from the existing species which is termed Eustephanus 

 galeritus. On the island it met with very changed but 

 highly favourable conditions an abundant shrubby vegeta- 

 tion and a tolerably rich flora ; less extremes of climate than 

 on the mainland ; and, most important of all, absolute freedom 

 from the competition of rival species. The flowers and their 

 insect inhabitants were all its own there were no snakes or 

 mammalia to plunder its nests ; nothing to prevent the full 

 enjoyment of existence. The consequence would be, rapid 

 increase and a large permanent population, which still main- 



1 See Macmillan's Magazine, September 1867, " On the Colours of Animals 

 and Plants," and chapters v. and vi., post. 



