928 ECOLOGY 



this is usually a matter of small moment in determining the ultimate 

 population."^ Geological history shows that the endemism of Torreya, 

 noted above, is in no wise due to disseminule immobility, for it was once 

 widely distributed, a fact that suggests that the most important of all 

 factors in distribution may be the fitness of a species to exist under the 

 given conditions. 



The origin of seed structures. — Nothing is known concerning the 

 factors involved in the origin of the manifold features of seeds and fruits 

 which fit them for the role of disseminules. It has been suggested that 

 these features have arisen through natural selection, but such a hypothe- 

 sis seems incredible in view of the obvious difficulty in grouping seed 

 plants in the order of successf ulness in such a way as to show a definite 

 relation to their kinds of disseminules. Even in the annuals, which 

 depend most upon seeds, there is no obvious relation in most cases be- 

 tween mobility and success. Nor is anything definitely known as to the 

 factors involved in seed formation, except that it is the final process of 

 the series initiated by flower formation, which has been seen to be facili- 

 tated by xerophytic conditions and by poor nutrition. The seed is by 

 far the most xerophytic structure of the entire series, and thus may bear 

 a definite relation to the causative factors of the reproductive processes. 



The planting of seeds. — While gardeners are particular as to the 

 depth at which seeds of various sizes are planted, there is no such sorting 

 in nature. Large and small seeds alike fall to the ground and gradually 

 become covered by falling leaves, by decaying herbage, or by soil that 

 is deposited by winds or waters. Doubtless many small seeds become 

 buried too deeply to permit of successful germination; such a fate is 

 rarer vdth large seeds, except, perhaps, where they are covered by the 

 deep alluvium of streams. While superficial planting doubtless is more 

 favorable for small seeds than for large seeds, the latter may none the less 

 germinate successfully at the surface; perhaps the chief danger in the 

 shallow planting of large seeds is that there may not be sufficient water 

 for germination. 



transportation seem to have made up, in the case of many species, for any natural lack of 

 disseminule mobility. 



1 Where similar habitats are discontinuous, as in oceanic islands, the flora may be 

 made up for a much longer time than elsewhere of plants with mobile disseminules; 

 the preponderance of ferns in many such places probably is thus explained. Vet even on 

 Krakatoa, a quarter of a century has been long enough for the invasion of a number of 

 species with apparently immobile disseminules, whose mode of migration is unknown. 

 It is to be noted that one seed, however extraordinary its mode of migration, may be 

 sufficient to populate a new area with an abundant vegetation. 



