384 STUDIES, SCIENTIFIC AND SOCIAL chap. 



would need to be differentiated. Neither is the small 

 amount of divergence that usually prevails between the 

 mean of a few individuals taken at random, such as might 

 have accidentally reached an island, and the average type 

 of the species, at all comparable with the well-marked 

 characters that usually distinguish insular forms, and 

 there is nothing in mere isolation without selection which 

 can increase the difference. As examples we may refer to 

 the many peculiar species of butterflies and birds found 

 in the various islands of the West Indian and Malayan 

 Archipelagoes, which are quite as distinct from each other 

 as are allied continental species, and which exhibit all the 

 characteristics of species which have been fully differ- 

 entiated by natural selection.^ 



^ As this point is still continually misunderstood by writers of 

 considerable ability, it will be well to give here the substance of 

 a reply I have made to their objections in Nature of January 12th, 

 1899. 



The late Dr. St. George Mivart (in the Journ. Linn. Soc. Zoology, 

 No. 172,) gives numerous cases of species of Lories peculiar to various 

 Papuan or Pacific Islands, which differ in some details of coloration 

 from the nearest allied species in other islands. He argues, as Captain 

 Hutton had previously argued in the exactly parallel case of the fruit- 

 pigeons of the genus Ptilopus, that these various specific markings 

 cannot be useful as "recognition-marks" because the colour and 

 marking of each of these genera is so very distinct from those of all other 

 birds inhabiting the same island, there being usually only one species 

 of a genus in each island. This conclusion is, however, not justified 

 by the whole circumstances of the case, nor by the fundamental 

 principle of organic evolution (-which both these writers admit) — that 

 each species has actually been developed from some allied species, 

 living or extinct. Let us consider therefore what are the actual 

 conditions of the problem. It is clear that each species of Lory 

 or Fruit-pigeon now found isolated in any of the islands onust (if evolution 

 be admitted) have originated by modification from some parent species. 

 The modification must have occurred in one or other of the modes 

 suggested in the text, and the only mode that implies any special 

 difficulty is when the species originated in the island where it is 

 now found by the modification of some other species which accidentally 

 reached the island. Let us therefore follow the process of change 

 step by step. 



The first thing that happens on the introduction of a few individuals 

 of a species new to an island which is well suited to it, and where 

 there are no other enemies than those to which it is already adapted, 

 is its rapid increase in numbers till the island is fully stocked. That 

 this happens we know from the increase of the rabbit in Australia, 

 New Zealand, and Porto Santo, the sparrow in the United States, 

 and numerous other cases. But as soon as the island is fully stocked 



