A SINGLB PAIR AND THE AVERAGE CHARACTER. 251 



some other adjustments. This is, I think, a very interesting point, 

 as it suggests how it is that, in some cases at least, physiological diver- 

 gence of this kind is one of the first forms of divergence that arises. 

 But in some species other adjustments seem to be more delicate than 

 this, and, therefore, more easily disturbed, while in others several 

 sets of adjustments, as colors and other recognition marks with the 

 preferences that correspond, and the habits of feeding and defense, 

 are in a state of equilibrium, the stability or instability of which is 

 about the same as of that which determines the relations of the male 

 and female elements. In this last class of cases several forms of 

 divergence may arise during the same stage of development, and that, 

 too, when the isolated portions are exposed to the same environment. 

 In some species a large number of characters are in a state of unstable 

 adjustment. As Professor Lankester has suggested near the close 

 of his review of Wallace's book, this cause of divergence seems to be 

 specially operative in the case of human faculties. But variability 

 with plasticity of type is not the only condition that affects the sta- 

 bility of segregated portions of a species. Other things being equal, 

 a single pair of any species is much less likely to represent the average 

 of all the characters of the species than a million pairs. This consid- 

 eration throws light on the comparative lack of divergence between 

 the land animals of England and those of Ireland, which lack has been 

 referred to by Mr. Wallace as an objection to my theory. In this 

 case, many millions of some of the species were probably existing in 

 each district at the time of the separation. As Professor Lankester 

 has pointed out, the representatives of the human species in the two 

 districts have somewhat diverged; and the probability is that, if 

 we were equally acquainted with the other species, we should find 

 other examples of divergence in minor points. If the isolation is 

 made more complete, and is longer continued, I believe the diver- 

 gence will gradually become more apparent. 



Mr. Wallace has mentioned another class of divergences, which he 

 has explained as due to surplus energy in the species, ready for expen- 

 diture in ways that are not determined by conditions in the environ- 

 ment. I maintain that through unstable adjustment this surplus 

 naturally takes different forms when the species is divided into iso- 

 lated groups ; and Wallace is content to attribute the divergence to 

 individual variability, though each group maintains its own type. 

 His words are : 



The enormously lengthened plumes of the bird of paradise and of the peacock 

 must be rather injurious than beneficial in the bird's ordinary life. The fact that 

 they have been developed to so great an extent in a few species is an indication 



