154 NATURAL SELECTION vi 
also in the forms given to living organisms” (Reign of Law, 
. 248). 
. ie the statement that “no connection can be conceived 
between the splendour of the humming birds and any function 
essential to their life,” is met by the fact that Mr. Darwin 
has not only conceived but has shown, both by observation 
and reasoning, how beauty of colour and form may have a 
direct influence on the most important of all the functions of 
life, that of reproduction. In the variations to which birds 
are subject, any more brilliant colour than usual is believed 
to be attractive to the females, and would therefore lead to 
the individuals so adorned leaving more than the average 
number of offspring. There are some indications that this kind 
of sexual selection does actually take place, and the laws of 
inheritance would necessarily lead to the further development 
of any individual peculiarity that was attractive, and thus the 
splendour of the humming birds is directly connected with 
their very existence. It is true that “a crest of topaz may 
be no better than a crest of sapphire,” but either of these may 
be much better than no crest at all; and the different condi- 
tions under which the parent form must have existed in 
different parts of its range will have determined different 
variations of tint, either of which were advantageous.1 The 
reason why female birds are not adorned with equally brilliant 
plumes is sufficiently clear; they would be injurious by ren- 
dering their possessors too conspicuous during incubation. 
Survival of the fittest has therefore favoured the development 
of those dark green tints on the upper surface of so many 
female humming birds, which are most conducive to their 
protection while the important functions of hatching and 
rearing the young are being carried on. Keeping in mind 
the laws of multiplication, variation, and survival of the 
fittest, which are for ever in action, these varied develop- 
1 Since writing this essay I have come to the conclusion that mere 
diversity of colouring between species is an important factor in their differ- 
entiation, serving as a means of recognition, and thus preventing cross- 
unions, See Darwinism, p. 217. Ihave also been led to doubt the reality 
of the fact of female selection of slight differences of colour on which Mr. 
Darwin relied, but it has not been thought advisable to alter the passages 
which seem to admit it, as they represent my belief at the time they were 
written, 
