198 THE HOLLOW CURVE OF DISTRIBUTION [pt. ii 



continents, and to the whole world (p. 185). It shows, and that 

 very conspicuously, in the composition of the various families 

 by sizes of genera (p. 186), as well as in the average size of the 

 largest genus in each family (taking the families in order of size, 

 p. 188). It shows in the four curves of percentages given on 

 p. 181. And it shows, finally, with great regularity of expression 

 in the curve for all the genera of flowering plants grouped by 

 sizes (p. 185), and in other features. There is no limit to the 

 number of instances that could if needful be produced. 



Now this is really a very remarkable state of affairs, and that 

 it has not been discovered at a much earlier period can only be 

 attributed to the fact that the rise of the theory of natural 

 selection diverted effort from the lines which it is clear (cf. p, 3) 

 that it was beginning to follow in 1853. Until, however, the 

 theory of evolution Avas firmly established, it seems doubtful if 

 much could have come of any demonstration of the effects of 

 age. The clear arithmetical relationships that exist between the 

 various groups of plants, "Avides"' and endemics for example, are 

 only explicable if one consider that they are mutually related. 

 The Darwinian theory established for us the law of evolution, 

 and it now remains to carry the Avork a stage further. 



It is somewhat difficult to perceive why the noAv clearly 

 demonstrated fact, that age is the most poAverful clement in the 

 dispersal of species, should rouse so much opposition. That an 

 older species should occupy more area than a younger one that is 

 closely related to it, seems almost axiomatic, and Avas cA^dently 

 clearly recognised by Lyell and Hooker (cf. p. 3). If tAvo species 

 A and B have much the same dispersal methods, and are suited 

 to much the same soils and climates, then it is clear that if Ave 

 call these three factors a, b, and c. the dispersal of these tAvo 

 species Avill be represented by the formula: 



dispersal = {a + b + c) x age. 



If the dispersal is the same, therefore, the age Avill be about the 

 same, while if the dispersal of A is greater than that of B, its age 

 Avill be greater. If avc transfer age to the left-hand side of the 



equation, avc get - - = a + b + c. shoAving that dispersal 



age 

 goes AA'ith age only. But age simply represents the total effect 

 of the operative factors a, b, and c, Avhich Avill be the greater the 

 longer the time during Avhich they have been acting. 



For the last half century, however, we have been under the 



