126 TEST CASES [ch. xi 



advantage of dispersal to some little distance — an almost certain 

 advantage if not pressed too far — how were the non-winged 

 parents killed out, unless the winged offspring were also superior 

 to them in some functional character that enabled them to kill 

 out the parents upon ground which they already occupied, and 

 where they had the great advantage given by the fact that they 

 were already established there, and that transport of seeds in 

 windless forest was a very difficult thing? 



TEST CASE XVI. THE ORIGIN 

 OF LARGE GENERA 



Another troublesome problem for the selectionist is to explain 

 how the method of selection gave rise to large genera. Upon what 

 grounds of adaptation did Senecio come to have about 3000 

 species, and other genera also have enormous numbers, com- 

 bining with the numbers a vast distribution over the earth's 

 surface? If they owe their wide dispersal ("success") to adapta- 

 tion, that adaptation can only have been generic. There are no 

 characters in the individual species that one can point to as 

 adaptive, and how could an adaptive and generic feature be 

 produced in a genus formed from below upwards by the dying 

 out of intermediates between it and its near relatives? If one of 

 the species that were going to form Senecio had a really fine 

 adaptation, one would expect it to go ahead and rather form a 

 genus of its owti than simply join the rest. The bulk of the species 

 in these big genera are local in distribution, and it is far simpler 

 to explain the whole matter by differentiation and by age, which 

 simply says that on the average the wider-dispersed species are 

 the older. 



Other remarks on "generic" adaptation will be found on 

 pp. 18, 59. 



TEST CASE XVII. SOME MORPHOLOGICAL 



PUZZLES 



Even in the comparatively few cases where a plant shows some 

 structural feature that may be looked upon as a definite physio- 

 logical advantage, like the tentacles of the Droseraceae, natural 

 selection is hard put to it to explain how they could be formed by 

 gradual adaptation. How, for example, did it produce the mar- 

 vellously sensitive tentacles of Drosera itself, when the first steps 



