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ii EQUATORIAL VEGETATION 267the numbers of suitable insects are totally inadequate to thefertilisation of the countless millions of forest trees over suchvast areas as the equatorial zone presents, and that, in consequence,a large proportion of the species have becomeadapted either for self-fertilisation, or for cross-fertilisation bythe agency of the wind. Were there not some such limitationas this, we should expect that the continued struggle forexistence among the plants of the tropical forests would haveled to the acquisition, by a much larger proportion of them,of so valuable a character as bright -coloured flowers, thisbeing almost a necessary preliminary to a participationin thebenefits which have been proved to arise from cross-fertilisationby insect agency.Concluding Remarks on Tropical VegetationIn concluding this general sketch of the aspects of tropicalvegetation, we will attempt briefly to summarise its mainfeatures. The primeval forests of the equatorial zone aregrand and overwhelming by their vastness, and by the displayof a force of development and vigour of growth rarely ornever witnessed in temperate climates. Among their bestdistinguishing features are the variety of forms and specieswhich everywhere meet and grow side by side, and the extentto which parasites, epiphytes, and creepersfillup every availablestation with peculiar modes of life. If the travellernotices a particular species and wishes to find more like it,hemay often turn his eyes in vain in every direction. Trees ofvaried forms, dimensions, and colours are around him, but herarely sees any one of them repeated. Time after time hegoes towards a tree which looks like the one he seeks, buta closer examination proves it to be distinct. He may atlength, perhaps, meet with a second specimen half a mile off,or may fail tillaltogether, on another occasion he stumbleson one by accident.The absence of the gregariousor social habit, so generalinthe forests of extra-tropical countries, is probably dependenton the extreme equability and permanence of the climate.Atmospheric conditions are much more important to thegrowth of plants than any others. Their severest strugglefor existence is against climate. As we approach towards