2G4 PKESIDEXTIAL ADDRESS. 



Francis Gallon has, in another connection, summarized the 

 effects of this cessation of selection in the convenient phrase 

 " Regression to Mediocrity." This is the Panmixia of Pro- 

 fessor Weismann and his followers ; but the phrase regression 

 to mediocrity through cessation of selection appears to me 

 preferable. It is clear that so long as any organ or struc- 

 ture is subject to natural selection through elimination, it 

 is, if not actually undergoing improvement, kept at a high 

 standard of efficiency through the elimination of all those 

 individuals in which the organ in question falls below the 

 required standard. But if, from change in the environment 

 or other cause, the character in question ceases to be subject 

 to selection, elimination no longer takes place, and the high 

 standard will no longer be maintained. There will be rever- 

 sion to mediocrity. The probable amount of this reversion 

 is at present a matter under discussion. Unless the 

 principle of atavism is called into play to an extravagant 

 extent, it does not seem probable that the reversion would be 

 large in amount. Pive per cent, would appear to be a very 

 liberal estimate. In any long-established character, such as 

 wing-power in birds, brain development, the eyes of Crus- 

 tacea, no shortcomer in these respects would have been per- 

 mitted by natural selection to transmit his shortcomings 

 for hundreds of generations. All tendency to such short- 

 comings would, one would suppose, have been bred out of 

 the race. If after this long process of selection there still 

 remains a strong tendency to deterioration, this tendenc}^ 

 demands an explanation, unless we call into play reversion 

 to ancestors dimly ancestral. The matter, however, should 

 not be left to the arbitration of a priori reasoning. Definite 

 experiments should be instituted to determine by observa- 

 tion the amount of regression. If the tendency to deteriora- 

 tion be strong, it should be found that the mean valii3 of tho 



