xxxiv GENERAL ARGUMENT AND CONCLUSION 521 



Selala is derived from the Asiatic species (R. mucronata), not as 

 the result of a cross but as connected with its dimorphism ; and 

 in support of this it is pointed out that on the Ecuador coast of 

 South America, where only the American species exists, a dimor- 

 phism is also displayed, one of the forms approaching in several 

 of its characters the Fijian Selala, though fruiting abundantly and 

 bearing the impress of a closer connection with the typical 

 American species than with the Asiatic plant. The view that 

 Rhizophora mangle reached the Western Pacific from America is 

 rejected, and it is considered that this species was originally as 

 widely diffused in the Old World as in America, and that it now 

 survives only in a few places in the tropics of the Old World. The 

 results of detailed observations on the modes of dispersal and on 

 the germinating process both with Rhizophora and Bruguiera are 

 given ; and the absence, as a general rule, of any period of rest 

 between the fecundation of the ovule and the germination of the 

 seed is established. 



A special chapter is devoted to the significance of vivipary, and 

 it is considered that a record of the history of vivipary on the globe 

 is afforded in the scale of germinative capacity that begins with the 

 seedling hanging from a mangrove and ends with the seed that is 

 detached in an immature condition from an inland plant. It is 

 suggested that with the drying up of the planet in the course of 

 ages the viviparous habit, which was once nearly universal, has been 

 for the most part lost except in the mangrove swamp, which to 

 some extent represents an age when the earth was enveloped in 

 cloud and mist and the atmosphere was saturated with aqueous 

 vapour. The lost habit is at times revived in the abnormal vivi- 

 pary of some inland plants, and traces of it are seen in the abnormal 

 structure of the seeds of some genera of the Myrtaceae, like Bar- 

 ringtonia, and in the seeds of genera of other orders. With the 

 desiccation of the planet and the emergence of the continents there 

 has been continual differentiation of climate resulting in sea- 

 sonal variation and in the development of the rest-period of the 

 seed. 



With the secular drying of the globe and the consequent 

 differentiation of climate is to be connected the suspension to a 

 great extent of the agency of birds as plant-dispersers in later 

 ages, not only in the Pacific Islands but over all the tropics. Tin- 

 changes of climate, bird, and plant have gone on together, the 

 range of the bird being controlled by the climate, and the distri- 

 bution of the plant being largely dependent on the bird. 



