252 APPENDIX III LETTERS PUBLISHED IN NATURE. 



of such perfect adaptation to the conditions of existence, such complete success 

 in the battle of Ufe, that there is, in the adult male at all events, a surplus of strength, 

 vitality, and growth power, which is able to expand itself in this way without injury. 

 That such is the case is shown by the great abundance of most of the species which 

 possess these wonderful superfluities of plumage. Why, in allied species, 



the development of accessory plumes has taken different forms, we are unable to say, 

 except that it may be due to that individual variabiHty which has served as the 

 starting point for so much that seems to us strange in form, or fantastic in color, 

 both in the animal and vegetable world.* (Darwinism, p. 293.) 



It is no small gratification to me that Mr. Wallace has found this 

 principle of unstable adjustment worthy of application to two impor- 

 tant classes of divergences; and that, in the case of one of these 

 classes, he has recognized that correspondence in such adjustments 

 can not be continuously maintained between the isolated portions of 

 a species. I trust that when he understands the relation in which 

 instability and isolation stand to each other in my theory he will 

 admit that it throws some light on the remarkable divergences of 

 Sandwich Island land mollusks. The subject was incidentally touched 

 upon in my paper on "Divergent Evolution through Cumulative 

 Segregation" (see Appendix I), and more fully discussed in the sup- 

 plemental paper on "Intensive Segregation" (see Appendix II). 



III. Indiscriminatb Separation, under the same Environment, a Cause oi 



Divergence, t 

 1. Divergence Resulting from Isolation. 



I have accumulated a large body of facts indicating that separated 

 fragments of a species, though exposed to the same environment, will 

 in time become divergent. I find that, wherever a species possessing 

 very low powers of migration is for many generations divided into a 

 series of fragments by barriers that do not obstruct the distribution 

 of surrounding species, more or less divergence arises in the separated 

 portions of the species, though, in the same areas, there is no diver- 

 gence in the environing species whose distribution is not obstructed. 

 I still further find that, whenever the distances intervening between 

 the different fragments are an approximate measure of the time and 

 degree of separate breeding (as is frequently the case as long as the 

 divergence does not involve any physiological and psychological 

 segregation), these distances are also an approximate measure of the 

 degree of divergence. 



The validity of this conclusion is called in question because it is 

 inconsistent with the theory that all divergence is due to diversity of 



* The italicizing is mine. 



t Published in Nature, August 14, 1890. 



