in SUDDEN VARIATION 151 



Belgian thinker, clearly expresses himself in favour of 

 the belief in a " tendency to betterment," some per- 

 manent cause with an unlimited aim, which will 

 operate as long as our planet lasts, and as long as 

 something may be imagined which is more perfect 

 than that which exists. Clearly such a tendency is 

 undemonstrable. For even if things could be seen 

 to progress as they ought if this tendency exists, this 

 is no proof of the existence of the tendency. 



On the other hand, many naturalists appeal to 

 external influences in order to explain variation, and 

 they certainly can point to many facts which are of 

 weight, as we shall show further on. Conditions 

 which seem, at first sight, to have no importance, 

 often exert considerable influence on individuals or 

 groups of individuals. But a discussion of this matter 

 would require more time than can be afforded here, 

 and I must dismiss the subject after having briefly 

 stated the state of opinion concerning the cause of 

 variation. I only wish to add that while, in the earlier 

 period of Darwinism, much stress was laid on the 

 slowness with which variation appears and operates, 

 there is at present a tendency to a change of opinion. 

 This change has been well expressed in a recent paper 

 by W. H. Dall, On a Provisional Hypothesis of Saltatory 

 Evolution! The writer bases his argument on the oft- 



1 In this connection Mivart's Genesis of Species may also be cited. 



