132 THE THEORY OF MUTATION 



gradually slower and slower. At the end of the series 

 two sudden steps upward break the general regularity 

 of the series of records. But on examination of the 

 evidence it is found that these are associated with 

 special conditions, and are not really exceptional. The 

 first of these breaks that which occurs in 1892 is 

 coincident with the introduction of a new type of sulky, 

 having ball bearings and other improvements ; whilst 

 the record of 1903 was accomplished behind a pace- 

 maker carrying a wind-shield. Neither of these records, 

 therefore, is strictly comparable with the rest of the 

 series. 



The observations in this case do not, indeed, seem 

 to be sufficient to afford the basis for a final decision 

 against the theory of the indeterminate power of selec- 

 tion. Yet Malthus' criticism clearly applies very 

 definitely to such a case i.e., there must be a limit 

 beyond which the speed of the trotting-horse will 

 never improve without a fundamental change taking 

 place in his organization. It seems, therefore, safe to 

 conclude that the curve to which the series of recrods 

 approaches is of the character of a parabola i.e., one 

 which continually becomes more and more nearly 

 horizontal as the speed of the horse gradually ap- 

 proaches its highest possible limit. 



De Vries, then, contends that all new domestic breeds 

 have arisen by the discontinuous method as definite 

 novelties. Darwin himself was perfectly aware that 

 this is usually the case, but the conclusion which he 

 drew from the fact was a different one, as the following 

 passage shows : 



