ARE SPECIFIC CHARACTERS ALWAYS USEFUL? 255 



ciple when he attributes the disappearance of "rudimentary organs" 

 to the action of "panmixia." Now, in the cessation of reflexive selec- 

 tion which follows independent breeding, a similar principle is intro- 

 duced, and the inevitable result must be the weakening of the power 

 of heredity by which the portions of the species were held in corre- 

 spondence with each other before their separation. I have elsewhere 

 shown that separate breeding necessarily disturbs unstable adjust- 

 ments ; and we here see that the most stable of the adjustments by 

 which each part of a species is kept in correspondence with every 

 other part gradually becomes unstable under the continued influence 

 of separation. Whenever a species is divided into two portions that 

 do not interbreed, the forms of reflexive selection will cease to act 

 between the two portions, and they will continue in sexual, social, 

 and other forms of harmony with each other only in so far as the 

 force of the old heredity holds them to the old standards. But the 

 power of heredity in these respects will in time fail, and if the sep- 

 arate breeding is long continued, incompatibility in all these respects 

 tends gradually to arise. Moreover, it is manifest that incompatibil- 

 ity of industrial habits involving diversity in the forms of active (or 

 endonomic) selection will in time arise. I therefore maintain that 

 separation, which necessarily includes cessation of reflexive selection 

 between the portions separated, is a cause of segregation and diver- 

 gence ; and that this segregation is in time intensified by diversity of 

 environal selection, through diversity in the use of the environment. 



Unless the separated portions of a species possess exactly the same 

 average character (which we must believe is seldom, if ever, the case), 

 separation must, from the first, be more or less segregative ; and even 

 in cases where the portions completely correspond in character (if 

 there are any such cases), the cessation of reflexive selection which is 

 involved in the separate breeding must result in divergence as soon as the 

 power of heredity securing the original adjustments begins to weaken; and 

 this is in due time followed by other forms of intensive segregation. I 

 therefore conclude that indiscriminate separation may be regarded as 

 a preliminary form of segregation (that is, as demarcational segrega- 

 tion) and that intensive segregation cooperating with this produces 

 complete segregation. 



IV. THE UTILITY OF SPECIFIC CHARACTERS.* 



I have followed the discussion on the utility of specific characters 

 with great interest; and though I am at such a distance that my 

 thoughts may come a little late, I wish to call attention to a few points. 



* Published in Nature, April i, 1897. 



