ISOLATION 3S9 



wards as far as the Persian Gulf, where it meets and inter-breeds 

 with a whiter species (C. capellanus). A third species, C. 

 sharpii, inhabits Afghanistan, Turkestan and Siberia as far as 

 Tomsk. The area between that place and Krasnoyarsk — about 

 350 miles east — "is said," remarks Howard Saunders, "to be 

 occupied by hybrids between this bird and a large form of 

 Carrion Crow". 



The Game-birds and the Ducks furnish yet more numerous 

 instances, widely distinct species freely inter-breeding and pro- 

 ducing fertile hybrids, at any rate in captivity. Thus then the 

 theory of the infertility of closely allied species does not apply 

 in these cases. We may assume therefore that physiological 

 selection has played no very important part in the evolution of 

 these specific types. 



That segregation is sooner or later followed by changes in 

 the specific characters of a species, whereby it becomes trans- 

 formed into a species peculiar to the area in which it has 

 settled, cannot well be doubted, and a few instances may make 

 this clear. 



Thus, of the nineteen species of Cassowaries known to 

 science, one only occurs on the mainland of Australia, but 

 seven, it is to be noted, occur on the large island of New Guinea 

 — a fact which is not easy of explanation. The remaining 

 eleven species are found on as many separate islands — evidently 

 originally part of New Guinea — and represent one or other 

 of the parent New Guinea species, transformed apparently by 

 isolation. The Galapagos Islands tell the same story. Of 

 forty-two species of true land-birds all but one are peculiar to 

 these islands. But, be it noted, all are allied to birds, all the 

 descendants of birds, found on the mainland of tropical 

 America. These are but samples of cases where changes of 

 the external characters have followed apparently as a conse- 

 quence of isolation. Precisely similar changes are met with 

 among species where the isolating barriers are formed, not by 

 water, but by intervening land of a kind unsuited to the needs 

 of the species. Birds which live at high altitudes may serve us 

 as illustrations. Dr. Bowdler Sharpe, our greatest living author- 

 ity on the subject of the geographical distribution of birds, 

 was the first to draw attention to this peculiar fauna of high 

 altitudes ; which presents precisely the same phenomena as the 



