166 THE WORLD OF LIFE chap. 



other bird on the same island for which they could possibly 

 be mistaken. He then says : 



" Consequently it appears certain that most of these species were 

 developed singly, each in its own island. If this be the case, 

 the colours which now distinguish the different species cannot be 

 recognition-marks, because there is no other species in each island 

 with which they could be confounded." 



Shortly afterwards the late Dr. St. George Mivart made 

 the same objection as regards the very numerous species of 

 beautifully coloured lories which are found in all the islands 

 around New Guinea and in the Western Pacific. He urged 

 that the various peculiarities of colour cannot be useful as 

 recognition-marks, because the colour and markings of each 

 of the genera of these birds is so very distinct from that of 

 all other birds inhabiting the same island, and there is usually 

 only one species in each island. This argument, looked at 

 superficially, seems very strong, but it is not difficult to 

 show that it is a complete fallacy, if we follow out in 

 detail what must have occurred in each case. 



It is clear, admitting evolution (as both these writers did 

 admit it), that each of the species of pigeon or lory now 

 peculiar to an island must have originated from some 

 parent species in the same or some other island ; and there 

 are only two possible suppositions — either the species 

 originated in island A by modification of the present form, 

 and then migrated to island B, afterwards becoming extinct 

 in A ; or it migrated from A to B and became modified into 

 its present form in B. The latter case is by far the more 

 probable, and as it is clearly that which the critics contem- 

 plated, let us see exactly what must have happened. 



We know as a fact that, when any species reaches an 

 island or other new habitat for the first time, if the conditions 

 are favourable, it increases with marvellous rapidity, till the 

 island is fully stocked and the supply of food at some time 

 of the year begins to fail, or till some enemy — a rapacious 

 bird, for instance — finds out the rich banquet, and is soon 

 followed by others. The rabbit in New Zealand and Porto 

 Santo, the sparrow in the United States, and many others, 

 are examples of such rapid increase. But as soon as the 



