362 A HISTORY OF BIRDS 



Genus EnstcpJianus which are found nowhere else ; the only 

 other members of the genus occur in Chili and in Tierra del 

 Fuego ! The species on Juan Fernandez are quite distinct and 

 remarkable for the fact that the sexes are strikingly different, 

 while in the Chilian species they are very similar. Finally, 

 we have the peculiar Plover known as the " Wire-bird " {Aigia- 

 litis sanctcv-helencB), and found only on the small island of St. 

 Helena. This list might be still further extended, but sufficient 

 examples have surely, been cited to illustrate the effects of 

 isolation, or more properly the after effects thereof. Isolation 

 implies change of environment, and change of environment 

 means free play for factors hitherto held in check by the action 

 of Nature's selection peculiar to the environment vacated. 

 Sooner or later, as we have already remarked, natural selection 

 again comes into play, but not until, and unless, the liberated 

 factors assume a too luxuriant growth. But of this, more 

 presently. 



Finally, we may remark that the several species which have 

 so far been described are so many outlying members of various 

 Orders and Families, which have strayed into the highways 

 or backwaters of the world — as the case may be — from more 

 or less distant centres of distribution. And these centres are 

 to be found scattered over more or fewer larger areas known 

 as "zoo-geographical regions" (see Chapter IV.). 



Perhaps one of the most striking illustrations of Romanes' 

 discriminate isolation is that afforded by the curious Stork 

 known as the "Open-bill" {Anastonms). This genus is repre- 

 sented by two species whose chief peculiarity lies in the re- 

 markable form of the beak, the cutting edges of which are 

 separated for a great part of their length by a wide gap, but 

 this gap is met with only in fully adult birds. 



Of these two species one is African {Anastovms lanielli- 

 gerus) the other Indian {A. oscitans). The Indian species is 

 white relieved by black scapulars, primary coverts and quills ; 

 the tail is also black. The African species differs in many 

 important particulars. In the first place the gap of the beak 

 is much more marked, while the whole of the plumage is 

 black, even in the young birds. Furthermore, the feathers of 

 the back have undergone a curious change, terminating in 

 spines having a glossy, bronze-like appearance. 



