252 APPENDIX III—LETTERS PUBLISHED IN NATURE. 
of such perfect adaptation to the conditions of existence, such complete success 
in the battle of life, 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 variability 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 Islandland 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 IT). 
III. INDISCRIMINATE SEPARATION, UNDER THE SAME ENVIRONMENT, A CAUSE OF 
DIVERGENCE. f 
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. 
{ Published in Nature, August 14, 1890. 
