THE THEORY OF DESCENT 265 



only relate to degree or quantity and perhaps to numerical 

 conditions also, might have been " selected " out of given 



' O O 



contingent variations, if but one postulate could be regarded 

 as fulfilled. This postulate may appropriately be stated 

 as the fixation of new averages of variation by inheritance. 

 Let the average value of a variation, with regard to a 

 given property of a given species be n and let the value 

 n + m m being variable which is represented in fewer 

 individuals of course than is n, be such as to offer 

 advantages in the struggle for existence ; then the 

 individuals marked by n-\-m will have the greater chance 

 of surviving. Our postulate now states that, in order that 

 a permanent increase of the average value of the variation 

 in question may be reached, n + m in any of its variable 

 forms must be able to become the average value of the 

 second generation, as n was the average value of the first. 

 Out of the second generation again it would be the few 

 individuals marked by n -+- m + o, which would be selected ; 

 n -f- ?7i -f o would be the new average ; afterwards n -f- m -\- o 

 +p would be selected, would become the new average, and 

 so on. A black variety for instance might be selected by 

 such a series of processes out of a grey-coloured one without 

 difficulty. 



But our postulate is not beyond all doubt : certain 

 experiments, at least, which have been carried out about 

 the summation of variations of the true fluctuating 

 type by any kind of selection seem to show that there 

 may be a real progress for a few generations, but that 

 this progress is always followed by a reversion. Of course 

 our experience is by no means complete on this subject, 

 and, indeed, it may be shown in the future that positive 



