326 TROPICAL NATURE Iv 
variation, and with those other laws which I have endeavoured 
to show regulate the development of colour. 
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 constant, 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 
mammatlia 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. 
