Organic Evolution. 



191 



affect a reduction from the " survival-mean " to the " birth- 

 mean." This was referred to in the chapter on "Heredity 

 and the Origin of Variations," but may be again indicated. 

 Suppose the number of births among wild ducks be repre- 

 sented by the number nine, of which six are eliminated 

 through imperfections in the organs of flight. Let us 

 place the nine in order of merit in this respect, as is done 

 in the table on p. 172. The average wing-power of the 

 nine will be found in No. 5, there being four ducks with 

 superior wing-power (1 4), and four with inferior wing- 

 power (6 9). The birth-mean will therefore be at the 

 level of No. 5, as indicated to the left of the table. But if 

 six ducks with the poorest wings be eliminated, only three 

 survive. The average wing-power will now be found in 

 No. 2, one duck being superior and one inferior to it 

 in this respect. It is clear that this survival-mean 

 is at a level of higher excellence than the birth-mean. 

 Now, when the ducks are placed in a poultry-yard, 

 selection in the matter of flight ceases, and, since all 

 nine ducks survive, the survival-mean drops to the birth- 

 mean. We may variously estimate this retrogression ; but 

 it cannot be a large percentage I should suppose, in the 

 case under consideration, one or two per cent, at most. 

 But Professor Weismann says, " A continued decline from 

 the original degree of development must inevitably take 

 place." It is not evident why such decline should continue. 

 If variations continue in the same proportion as before, 

 the birth-mean will be preserved, since there are as many 

 positive or favourable variations above the mean as there 

 are negative or unfavourable variations below the mean. 

 A continuous decline must result from a preponderance of 

 negative over positive variations, and for this some other 

 principle, such as atavism, or reversion to ancestral 

 characters, must be called in. But in the case of so long- 

 established and stable an organ as that of flight, fixed 

 and rendered constant through so many generations, it is 

 hardly probable that reversion would be an important 

 factor. Mr. Galton has calculated that among human- 



