I INTRODUCTION 5 



of Samoa, Tahiti, and Hawaii in the Pacific Ocean, would be rightly- 

 regarded as evidence of active dispersal of the seeds through the 

 agency of birds from one mountain-summit to another, whether in 

 mid-ocean or in the centre of a continent. The prevalence of the 

 same beach-plants over most of the globe in the same climatic 

 zones would point unmistakably to the predominant agency of 

 currents. But with many plant-genera, some of which range the 

 world, whilst others again may be restricted to a single group 

 of islands in the Pacific, there is often no question either of 

 means of dispersal, or of station, or of plant-migration, and 

 problems of a very different nature are opened up. 



When we leave the beach and the mountain-top, the river and 

 the pond, all the troubles of distribution begin ; and since but a 

 small proportion of plants in a typical flora belong to these stations, 

 it follows that difficulties will dog our steps with the large majority 

 of the plants. The agencies of dispersal now working around us, 

 the current, the wind, the insect, the bird, and the bat, will explain 

 many of the features of littoral and alpine floras and of the vegeta- 

 tion of ponds and rivers. Here we have in so many cases wide- 

 ranging genera with the means of dispersal ready to hand. We 

 can connect the wide range of Vaccinium with the wide range of 

 birds of the grouse and other families that feed on the berries. 

 We can associate the great areas of aquatic or sub-aquatic genera, 

 like Potamogeton and Sparganium, with the migratory habits of 

 the ducks in the stomachs of which we find their seeds. We can 

 connect the great ranges of beach plants like Ipomea pes caprse 

 in the tropics, and Convolvulus soldanella in the temperate regions 

 with the currents, and the almost cosmopolitan range of many 

 ferns and lycopods with the winds and other agencies. 



When, however, we enter the forests we find genera that are 

 often much more restricted in their areas, and species that are yet 

 more limited in their range. There is very little dispersal going 

 on here. The birds are strange. Their distribution is usually 

 very local. They look lazily down at us from the branches, as 

 they disgorge the seeds and stones of the fruits they have eaten, 

 which cover the ground around. We can almost fancy that they 

 say : — " Our work is done. We rest from the toil of our ancestors. 

 They carried seeds to far-distant Hawaii, Tahiti, and Savaii. Our 

 work is done." And as we walk through those noiseless forests, 

 where the machinery of species-making is ever in silent motion, 

 we become aware that we are treading one of Nature's great work- 

 shops for the manufacture of species and genera. Outside the 



