70 



DIFFERENTIATION 



[CH. VIII 



A, as the oldest genus, should have the greatest area and the 

 greatest number of species, B the second, C the third, Bb probably 

 the fourth, and so on, but only on averages. Whilst in Ranuncu- 

 laceae Ranunculus has 325 species to 250 in Clematis, one would 

 hesitate, and rightly so, to say that the former was the older, when 

 one remembers that it is herbaceous, and Clematis shrubby. 

 As time goes on, it is clear that the rate at which new genera 



Fig. 7. Evolution by differentiation. Each genus is supposed to survive 



the whole way along the line at right angles to its origin, e.g. A still 



survives at H, B at Be, and so on. In order to save complication, the 



lines to show the gro^^i:h Bb, Be, &c. are not shown. 



are formed will increase. Each genus will begin with one species, 

 and after a time will form more, so that the few older genera of 

 the family will contain the greatest numbers of species. The result 

 will be (cf. 66, p. 185) the gradual formation of the familiar 

 hollow curve already described, with a few large genera of dif- 

 ferent sizes at the top, and many monospecific genera at the 

 bottom, the numbers increasing from top to bottom at an 

 accelerating rate. As there will rarely, upon this theory, be any 

 appreciable adaptational difference between species or even 

 genera, there will be little or no reason why the older ones should 

 be killed out (as there is under natural selection), and so the 

 increase in numbers will lead inevitablv to the hollow curve. 



