also have acted most efficiently in regions where the climatal condition remained constant, and where migration was unnecessary; while whatever direct effect may be produced by light or heat, will necessarily have acted more powerfully within the tropics. And lastly, all these causes have been in action over an actually greater area in tropical than in temperate zones; while estimated potentially, in proportion to its life-sustaining power, the lands which enjoy a practically tropical climate (extending as they do considerably beyond the geographical tropics) are very much larger than the temperate regions of the earth. Combining the effects of all these various causes we are quite able to understand the superiority of the tropical parts of the globe, not only in the abundance and variety of their forms of life, but also as regards the ornamental appendages and vivid coloration which these forms present. VI. THE COLOURS OF PLANTS AND THE ORIGIN OF THE COLOUR-SENSE. Source of Colouring-matter in Plants--Protective Coloration and Mimicry among Plants--Attractive Colours of Fruits--Protective Colours of Fruits--Attractive Colours of Flowers--Attractive Odours in Flowers--Attractive grouping of Flowers--Why Alpine Flowers are so Beautiful--Why allied species of Flowers differ in Size and Beauty--Absence of Colours in Wind-fertilized Flowers--The same Theory of Colour applicable to Animals and Plants--Relation of the Colours of Flowers and their Geographical Distribution--Recent Views as to the Direct Action of Light on the Colours of Flowers and Fruits--On the Origin of the Colour-sense--Supposed increase of Colour-perception within the Historical Period--Concluding Remarks on the Colour-sense.