U4 TROPICAL NATURE, AND OTHER ESSAYS. 



cation in form or structure, and we accordingly find that 

 the only important variations which have become per- 

 manent are those of size and of colour. The increased 

 size would naturally arise from greater abundance of 

 food with a more equable climate throughout the year, 

 the healthier, stronger, and larger individuals being pre- 

 served. The change of colour would depend on mole- 

 cular changes in the plumage accompanying the increase 

 of size ; and the superior energy and vitality in the 

 male, aided by the favourable change in conditions and 

 rapid increase of population, would lead to an increased 

 intensity of colour, the special tint being determined 

 either by local conditions or by inherited tendencies in 

 the race. It is to be noted that the change from green 

 to red is in the direction of the less refrangible rays of 

 the spectrum, and is in accordance with the law of change 

 which has been shown to accompany expansion in 

 inorganic, — growth and development in organic forms. ^ 

 The change of colour in the female, not being urged 

 on by such intense vital activity as in the case of the 

 male, would be much slower, and, owing probably 

 to inherited tendencies, in a different direction. The 

 under-surface of the Chilian bird is ashy with bronzy- 

 green spots on the breast, while the tail is entirely 

 bronze-green. In the Juan Fernandez species the under- 

 surface has become pure white, the breast-spots larger 

 and of a purer golden-green, while the whole inner web 

 of the tail-feathers has become pure white, producing a 

 most elegant effect when the tail is expanded. 



We may now follow the two sexes to the remoter 



1 See " Colours of Animals," Macmillan's Magazine, Sept. 1877, pp. 

 394-398, and Chapter V. in the present volume. 



