FIELD STUDY IN ORNITHOLOGY. • 479 



uevr residence became hotter in summer, tbey instinctively returned to 

 their birthplaces, and there reared their young, retiring with them 

 when the recurring winter im[»elled them to seek a warmer climate. 

 Those species which, nntitted for a greater amount of heat 'by their 

 more protracted sojourn in the northern regions, persisted in re-visiting 

 their ancestral homes, or getting as near to them as they could, retained 

 a capacity for enjoying a temperate climate, which, very gradually, 

 was lost by the species which settled down more iiernianently in their 

 new quarters, and thus a law of migration became established on the 

 one side, and sedentary habits on the other. 



MIMICRY. 



If there be one question on which the field naturalist may contri- 

 bute — as lion's provider to the philosoiiher more than another, it is on 

 the now much disputed topic of " mimicry," whether protective or 

 aggressive. As Mr. Beddard has remarked on this i>ubject, "The field 

 of hypothesis has no limits, and what we need is more study" — and, 

 may w^e not add, more accurate observation of facts. The theory of 

 protective mimicry was first propounded by Mr. H. W. Bates, from his 

 observations on the Amazon, lie found that the grcnip of butterfiies, 

 Heliconiida', conspicuously banded with yellow and black, were i^ro- 

 vided with certain glands which secrete a nauseating fluid, supposed 

 to render them unpalatal»le to birds. In the sand districts he found 

 also similarly colored butterflies, belonging to the family Pieridoc^ 

 which so closely resembled the others in shape and markings as to be 

 easily mistaken for them, but which, unprovided with such secreting 

 glands, were unprotected from the attacks of birds. The resemblance, 

 he thought, was brought about by natural selection for the protection 

 of the edible butterflies, through the birds mistaking them for the 

 inedible kind. Other cases of mimicry among a great variety of insects 

 have since been pointed out, and the theory of protective mimicry has 

 gained many adherents. Among birds, many instances have been 

 adduced. Mr. Wallace has described the extraordinary sinnlarity 

 between birds of very different families, OrioUis hountenslH and Phile- 

 mon moluccensis, both peculiar to the island of Bourn. Mr. H. O. Forbes 

 has discovered a similar brown oriole, Ori()Jii,s dccijyiens, as closely 

 imitating the appearance of the Philemon timorlaoenfiis of Timor-laut. 

 A similar instance occurs in Geram. But Mr. Wallace observes that, 

 while usually the mimicking species is less numeronsthan the mimicked, 

 the contrary appears to be the case in Bourn, and it is diflicult to see 

 what advantage has been gained by the mimicry. Now, all the species 

 of Philemon are remarkably somber colored birds, and the mimicry 

 can not be on their side. But there aie other brown orioles, more 

 closely resembling those named, in other Moluccan islands, and yet 

 having no resemblance to the Philemon of the same island, as may be 

 seen in the case of the Oriolus pha'ochromus and Philemon gilolensis 



