Ill] 



OF INHERITANCE OF SIZE 



269 



The physiological speculations we need not discuss: but, to take 

 a single example from morphology, we begin to understand the 

 possibihty, and to comprehend the probable meaning, of the all but 

 sudden appearance on the earth of such exaggerated and almost 

 monstrous forms as those of the great secondary reptiles and the 



500 



Pure Shetland 



10 20 



Age, in months 



30 



40 



Fig. 83. 



Effect of cross-breeding on rate of growth in Shetland ponies. 

 From Walton and Hammond's data.* 



great tertiary mammals f. We begin to see that it is in order to 

 account not for the appearance but for the disappearance of such 

 forms as these that natural selection must be invoked. And we 

 then, I think, draw near to the conclusion that what is true of these 

 is universally true, and that the great function of natural selection 



* Walton and Hammond, Proc. R.S. (B), No. 840, p. 317, 1938. 

 t Cf. also Dendy, Evolutionary Biology, 1912, p. 408. 



