CH. vj HINDER THE DISPERSAL OF SPECIES 35 



The differences in area occupied will continually increase (by 18 

 every time) though in radius of area B will always be only 3 

 behind ^. And in the same way, if each give rise to new species 

 in proportion to the area occupied {i.e. number of individuals) 

 A will continually gain upon B. In actual practice, of course 

 the result will not be so mechanical, but 07i the average the earlier 

 formed species will gain upon the later, both in area and in 

 number of progeny of new species, unless the later formed ones 

 are superior to the parents. This gain is incidentally shown to 

 be the case by looking over the geological record. The o-enera 

 that are found in the earliest horizons are in general large genera 

 of the present day. Twenty of them from one horizon, though 

 one or two are now extinct, include over 2000 species now living, 

 so that their average size is at present over eight times the 

 average of twelve species per genus. 



It is clear that in nature the usual case will be transport to a 

 small distance only. But when this has been accompHshed, the 

 seed has still to become a plant capable of reproducing itself, 

 and to do this it has to overcome manv difficulties, the chief 

 perhaps being the fact that, as a rule, the ground is already all 

 but completely occupied by plants more or less fully grown, so 

 that even a vacant space left by the death of one of them will 

 be full of roots, and overshadowed by the neighbouring plants. 

 Not only so, but the plants that grow upon any given piece of 

 ground in its natural state generally form Avhat is called an 

 association or society, into which a stranger, i.e. a plant of a 

 species not usually occurring in that association, will find entrv 

 very difficult. ^Ve shall return to this subject below. 



When a species is just commencing its life as such, and con- 

 sists possibly of a very fcM^ individuals, there is no doubt that 

 its chance of spreading, by seizing upon spots more or less vacant, 

 will be much less than when it becomes more common, as indi- 

 cated by the very few plants that make up many endemic 

 species (below, p. 55). A species, unless it start upon an un- 

 occupied piece of ground, will probably take a very long time 

 to spread from the condition of half-a-dozen plants on a few 

 square yards to reasonable frequency on a square mile. Once 

 established with commonness more or less equal to that of its 

 neighbours, it will probably spread with a rapidity much the 

 same as that of other species of the same genus living in the 

 same country and in the same type of vegetation, inasmuch as 

 all will probably have much the same type of mechanism for 



a— 2 



