INTRODUCTION. 
liii 
it is quite possible that such an amount of subsidence may have occurred. It is possible, too, that 
there may have been an extension northward to the Kermadec Islands, and even further to the Tonga 
and Fiji Islands, though this is hardly probable, or we should find more community between their 
productions and those of New Zealand. A southern extension towards the Antarctic continent at a 
somewhat later period seems more probable, as affording an easy passage for the numerous species of 
South American and Antarctic plants and also for the identical and closely allied freshwater fishes of 
these countries. The subsequent breaking up of this extensive land into a number of separate 
islands — in which the distinct species of Moa and Kiwi were developed — their union at a later period, 
and the final submergence of all but the existing islands, is a pure hypothesis, which seems necessary 
to explain the occurrence of so many species of these birds in a small area, but of which we have no 
independent proof” *. 
In a preceding section I have already mentioned that, as a rule, the species of Dinornis which, in 
former times, inhabited the North Island were different in character from their contemporaries in the 
South Island, although the two areas of land are only separated by a strait scarcely eighteen miles across 
in its narrowest part. The same feature is maintained to the present day in the existing Avifauna, clearly 
showing that each island has a biological history of its own. Thus the Saddle-back ( Creadion carun- 
culatus) of the North is represented in the South by C. cinereus, a closely-allied species, but differing 
in the colour of its plumage; Turnagra hectori (now almost extinct) is represented by T. crassirostris, 
a species that will soon follow suit, although still plentiful in certain localities ; the Weka ( Ocydromus 
earli ) is represented by several other closely-related species ( 0 . australis , O.fuscus, and 0. brachypterus) 
so closely resembling the northern bird both in appearance and habits that they are called “Wood- 
hens ” by the settlers of both islands and by them, as well as by the natives, are generally regarded as 
identical ; the Popokatea ( Clitonyx albicapilla) is represented by another species (C. ochrocephala ) 
differing in colour, but so closely allied to it that the Maoris apply the same name to both; the 
Toutouwai ( Miro australis), to which precisely the same remark applies, is represented by M. albi- 
frons, and Glaucopis wilsoni by G. cinerea, distinguishable only by the colour of its ornamental 
wattles. Another case in point is furnished by the two representative species of Apteryx, the North 
Island bird being characterized by a different structure of plumage to that of the well-known Apteryx 
australis inhabiting the South Island. Till of late years it was believed that Apteryx oweni, which 
differs entirely from both of these species in the grey colour and mottled appearance of its plumage, 
was confined to the colder districts of the South Island; but in 1876 I communicated to the Wel- 
lington Philosophical Society the discovery of this bird near the summit of the Tararua mountains 
on the north side of Cook’s Strait, where it was found frequenting the stunted vegetation immediately 
below the snow-line f . The existence of this species was entirely unknown to the Maoris of the 
North Island, and its occurrence under the conditions I have mentioned is a very interesting fact in 
geographical distribution. 
Analogous cases of representative species in more or less widely separated areas are of frequent 
occurrence in other parts of the world. “ The cause of this ” (writes Mr. Wallace) “ is very easy to 
understand. We have already shown that there is a large amount of local variation in a considerable 
number of species, and we may be sure that were it not for the constant intermingling and inter- 
crossing of the individuals inhabiting adjacent localities this tendency to local variation would soon 
form distinct races. But as soon as the area is divided into two portions, the intercrossing is stopped, 
* 1 Island Life,’ p. 454. f Trans. N.-Z. Instit. vol. viii. pp. 193-194. 
