CHAPTER V 



CAUSES WHICH FAVOUR OR HINDER 

 THE DISPERSAL OF SPECIES 



It being generally considered that a species commences upon a 

 comparatively small area (or areas), it is clear that it has to do 

 much travelling to cover the large territory which is now occu- 

 pied by so many forms. In general it will be dispersed by aid 

 of one of the methods already described, whether regular or 

 not, and will be aided, or far more often hindered, in its 

 journeyings by various factors which we have now to consider. 



Whatever one's view^s may be as to the efficacy of transport 

 to a distance, it is unquestionable that as a rule new plants of 

 a given species grow up fairly near to pre-existing specimens of 

 the same kind. For one thing, though it is often overlooked, it 

 is much more difficult for a plant carried to a distance to estab- 

 lish itself, under the different conditions of climate, soil, and 

 especially of plant societies, etc., that it will then meet with, 

 than if it were simply transported a few yards, just as it would 

 be more difficult for an emigi-ant from England to establish 

 liimself in a foreign country, rather than in a colony or the 

 United States. 



If near to a solitary tree of a given kind there exist, at dis- 

 tances of ten, a hundred, and a thousand yards, spaces where 

 its seeds if sown w^ould stand a reasonable chance of growing 

 and flourishing, then it is clear that to put a seed on every 

 square yard^ up to a distance of ten (supposing the impossible 

 case of uniform distribution), the tree would have to disperse 

 314 seeds; up to 100 yards 31,400; and up to 1000 yards it 

 would need no less than 3,140,000 seeds, a number probably 

 far beyond the capacity of most trees. In actual fact, the seeds 

 are notoriously carried in such vastly greater numbers to the 

 smaller distances, that this figure would probably have to be 

 multiplied by 100, or even 1000 or more, to allow of the plant 

 placing a seed on every square yard up to a radius of 1000 yards. 

 But even this is not enough, for a seed placed in one part of a 

 square yard, while the suitable spot for its growth is in another, 

 will have no better chance of success than if a dozen yards away. 

 1 Area of circle = Trr", e.g. 314 x 10 x 10. 



