If we admit preferential breeding as a segregation-factor (and arising out of it sexual selection, in a modified form, as a determining one in the evolution of the plumage of male birds), it is evident that the standard of recognition-marks can only be maintained by a uniformity of preference or taste. Still, the uniformity is not likely to be absolute. In this matter, as in others, variations will occur, and after the lapse of a thousand generations, in which elimination has been steadily at work, it is hardly probable that the recognition standard would remain absolutely unchanged. For, though there may not be any direct elimination in this particular respect, there might well be colour-eliminations in other (e.g. protective) respects, and the mental nature would not remain quite unchanged. Moreover, we know that secondary sexual characters are remarkably subject to variation, as may be well seen in the case of ruffs (_Machetes pugnax_) in the British Natural History Museum. In the case of our two islands with isolated faunas, therefore, if they formed separate breeding-areas for birds, the chances would be many to one against the change in the standard of recognition-marks being identical in each area. Hence might arise those minute but definite specific distinctions which are so noteworthy in this class of the animal kingdom. Instance the Old and New World species of teal, the Eastern and Western species of curlew and whimbrel, and other cases numerous.[AX] This, in fact, is probably in many cases the true explanation of the occurrence of representative species, slight specific variations of the same form as it is traced across a continent or through an archipelago of islands.