INTRODUCTION. 5 



rock, and even from the yet recent cinders and lava of the 

 volcano, Nature prepares the way for vegetable existence. The 

 slightest crevice or inequality is sufficient to arrest the invisible 

 germs that are always floating in the air ; and the humble plants 

 which spring from these soon overspread the surface, deriving 

 their chief nutriment from the atmosphere. Having completed 

 their allotted period of existence, they die and decay ; but their 

 death is only a preparation for the appearance of higher forms of 

 vegetable structure. They are followed by successive tribes of 

 plants, of gradually increasing size and strength ; until, in the 

 course of years, the sterile rock is converted into a natural and 

 luxuriant garden, of which the productions, rising from grasses 

 to shrubs and trees, present all the varieties of the fertile meadow, 

 the tangled thicket, and the widely-spreading forest. 



No extremes of heat or cold seem to put an entire check upon 

 vegetation. Even in the desert plains of the torrid zone, the 

 eye of the traveller is often refreshed by the appearance of a few 

 hardy plants, which find sufficient materials for their growth in 

 these arid regions. And wherever a spring of water moistens the 

 soil and atmosphere around, a spot of luxuriant verdure is found. 

 These Oases, as they are termed, are the stations at which caravans 

 halt, when crossing the extensive wastes of parching sand ; and 

 although their effect upon the mind is doubtless heightened by 

 the dreariness of the preceding journey, there is no question that 

 few spots can present greater richness of vegetation than these. 

 It will be seen, hereafter, that heat, light, and moisture form 

 the circumstances most favourable to the growth of plants ; and 

 it is from the combination of the latter of these conditions with 

 the former, that the vegetation of small islands in the tropical 

 ocean is so peculiarly rich. These Oases are like such islands in 

 the midst of a sea of sand ; and nothing can be a greater contrast 

 with the desolation around, than " the green pastures" and " still 

 waters" which they afford. 



Many remarkable facts might be mentioned, relative to the 



