VARIED FORMS ON DIFFERENT ISLES 275 



climatic conditions ; the factor that varies, however, is 

 the proportion of species of Planema. The supposition 

 is that on Ngamba a form of Pseudacraea that does not 

 resemble male or female of Planema macarista has 

 less chance of surviving birds' attacks than a form which 

 is deceptively like macarista ; consequently the form terra, 

 which is so common on Kome Isle only a few hundred 

 yards away, is at a disadvantage on Ngamba. 



This explanation, of course, presupposes that the 

 species of birds which are presumed to exercise selection 

 do not fly across from Ngamba to Kome or vice versa 

 (see map). Now, although Bee-eaters may constantly 

 be seen crossing open water in flocks, I do not 

 think that I have ever seen such birds as Flycatchers, 

 which abound on the islands and, in some cases at least, 

 only in the forest which is the haunt of Pseudacraeas, 

 crossing from one island to another. I have even noticed 

 that the very characteristic songs and call notes of species 

 such as the " Kunguvu " (Tchitrea emini) and a pretty 

 little black and white Platysteira (? jacksoni) varied on 

 different islands. 



While on Bugalla in 1912-13, I became extremely 

 familiar with the call of these abundant birds, and 

 directly I visited the Kome group of isles in 1914 at 

 once noted a difference — yet the difference was slight 

 enough to make me feel certain that the species of 

 bird was the same. This seems to show that at any 

 rate on islands at this distance apart there is a tendency 

 for geographical races to be perpetuated, but I have 

 not been long enough among the islets of the Kome group 

 to know if the song of the flycatchers of Ngamba differs 

 from that of the same species on Kome. Yet, as has 

 been shown in Chapter VI, many islands quite close 

 together show distinct differences in their avian fauna. 



