34 CAUSES WHICH FAVOUR OR [pt. i 



The area would thus grow with increasing speed\ and though 

 for a long time it would be A^ery small, at the end of a million 

 years the radius Avould be 1 million yards, or roughly 600 miles 

 (London to the Shetlands, Dresden, and the Pyrenees), and the 

 area over a million square miles. 



Thus, in a period of time which is almost insignificant from 

 a geological point of view, a species without competition or 

 interference might cover an area which is probably larger than 

 the average area of a species to-day. At the same rate of travel, 

 in 12 million years it would cover an area of 50 million square 

 miles, equal to the whole available land-surface of the globe, 

 and in 24 million years might cover the entire surface of the 

 earth, supposed land with uniform conditions. All these periods 

 are probably small compared even to the Tertiary period of the 

 earth's history, for Lord Rayleigh has estimated the time since 

 the Eocene alone at 30 millions. 



These figures are of course the merest rough approximations, 

 and are given simply to show how little actual forward move- 

 ment is required to do, in a comparatively short space of time, 

 what has actually been done by even the most widely distributed 

 species. No special mechanism for dispersal would be imperative 

 in such a case. It is clearly obvious that in nature what actually 

 happens must be delay of spread rather than acceleration. 



Another important point that one must not allow to be for- 

 gotten, and which may perhaps be dealt with best in this place, 

 is the simple arithmetical ratio in which an early species will 

 gain upon one that appears at a later period, both in the area 

 occupied, and in the chance of giving rise to new species. Let 

 us suppose that both of these are purely mechanical processes, 

 and that the species spread uniformh^ in every direction, as 

 before, without let or hindrance. Then if two species A and B 

 start at different periods, spreading at the same rate, B will 

 never catch up to A, but Avill always fall behind. The areas 

 occupied will be (cf. above): 



^ 3 12 27 48 75 108 147 192 

 B — — — 3 12 27 48 75 



Difference... 45 63 81 99 117 



1 The dispersal would of course tend to become less and less dense, but 

 as for Age and Area purposes area is estimated by drawing a circle round 

 the outermost localities, this matters little. 



