September 21, 1893] 



NA TURE 



495 



migration, there yet remains a vast untilled region for the field 

 naturalist. 



What Prof. Newton terms the sense of direction, unconsciously 

 exercised, is the nearest approach yet made to a solution of the 

 problem. He remarks how vastly the sense of direction varies 

 in human beings, contrasting its absence in the dwellers in towns 

 compared with the power of the shepherd and the countryman, 

 and, infinitely more, with the power of the savage or the Arab. 

 He adduces the experience of MiddendorfT among the Samo- 

 jeds, who know how to reach their goal by the shortest way 

 through places wholly strange to them. He had known it among 

 dogs and horses (as we may constantly perceive), but was sur- 

 prised to find the same incomprehensible animal faculty un- 

 weakened .imong uncivilised men. Nor could the Samojeds 

 understand his enquiry how they did it ? They disarmed him 

 by the question. How now does the arctic fox find its way aright 

 on the Tundra, and never go astray ? and Middendorff adds : 

 " I was thrown back on the unconscious performance of an in- 

 herited animal faculty "; and so are we I 



There is one more kind of migration, on which we know 

 nothing, and where the field naturalist has still abundant scope 

 for the exercise of observation. I mean what is called excep- 

 tional migration, not the mere wanderings of waifs and strays, 

 nor yet the uncertain travels of some species, as the crossbill in 

 search of food, but the colonising parties of many gregarious 

 species, which generally, so far as we know in our own hemi- 

 sphere, navel from east to west, or from south-east to north- 

 west. Such are the waxwing (Amptlis garnda), the pastor star- 

 ling {Pastor roseiis) and Pallas's sandgrouse, after intervals 

 sometimes of many years, or sometimes for two or three years 

 in succession. The waxwing will overspread VVestern Europe 

 in winter for a short time. It appears to be equally inconstant 

 in its choice of summer quarters, as was shown by J. Wolley in 

 Lapland. The rose pastor regularly winters in India, but never 

 remains to breed. For this purpose the whole race seems to 

 collect and travel noith-wesl, but rarely, or after intervals of 

 many years, returns to the same quarters. Verona, Broussa, 

 Smyrna, Odessa, the Dobrudscha have all during the last half- 

 century been visited for one summer by tens of thousands, who 

 are attracted by the visitations of locusts, on which they feed, 

 rear their young, and go. These irruptions, however, cannot be 

 classed under the laws of ordinary migration. Not less inex- 

 plicable are such migrations as those of the African darter, 

 which, though never yet observed lo the north of the African 

 lakes, contrives to pass, every spring, unobserved to the lake of 

 Anlioch in North Syria, where I found a large colony rearing 

 their young, which, so soon as their progeny was able to fly, 

 disappeared to the south-east as suddenly as they had arrived 



There is one possibleexplanationof the sense of direction uncon- 

 sciously exercised, which I submit asa working hypothesis. We are 

 all aware of the instinct, strong both in mammals and birds with- 

 out exception, which attracts them to the place of their nativity. 

 When the increasing cold of the northern regions, in which they 

 all had their origin, drove the mammals southward, they could 

 not retrace their steps, because the increasing polar sea, as the 

 arctic continent sank, barred their way. The birds reluctantly 

 left their homes as winter came on, and followed the supply of 

 food. But as the season in their new residence became hotter in 

 summer, they instinctively returned to their birthplaces, and 

 there reared their young, retiring with them when the recurring 

 winter impelled them to seek a warmer climate. Those species 

 which, unfitted for a greater amount of heat by their more pro- 

 tracted sojourn in the northern regions, persisted in revisiting 

 their ancestral homes, or getting as near to theui as they could, 

 retained a capacity for enjoying a temperate climate, which, 

 very gradually, was lost by the species which settled down more 

 permanently in their new quarters, and thus a law of migration 

 became established on the one side, and sedentary habits on the 

 other. 



If there be one question on which the field naturalist may 

 contribute, as lion's provider to the philo>opher, more than 

 another, it is on the now much disputed topic of " mimicry," 

 whether protective or aggressive. As Mr. Beddard has re- 

 marked on this subject, "The field of hypothesis has no 

 limits, and what we need is more study " — and, may we not 

 add, more accurate observation of facts. The theory of pro- 

 tective mimicry was first propounded by Mr. H. W. Bate?, 

 from his observations on the Amazon. He found that the 

 group of butterflies, Heliconiidcc, conspicuously banded with 

 yellow and black, were provided with certain glands 



NO. 1247, VOL. 48I 



which secrete a nauseating fluid, supposed to render' them un- 

 palatable to birds. In the sand districts he found also similarly 

 coloured butterflies, belonging to the family Picrida, which so 

 closely resembled the others in shape and markings as to be 

 easily mistaken for them, but which, unprovided with suih 

 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 simi- 

 larity between birds of very difTerent families, Oriolus hour- 

 iieiisis and Philemon moluuensis, both peculiar to the island of 

 Bouru. Mr. H. O. Korbes has discovered a similar bro*n 

 oriole, Oriolus decipiens^ as closely imitating the appearance of 

 the PhiUmon limorlaoensis of Timor-laut. A similar instance 

 occurs in Ceram. But Mr. Wallace observes that, while usually 

 the mimicking species is less numerous than the mimicked, the 

 contrary appears to be the case in Bouru, and it is difficult lo 

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

 the species oi Philemoit are remarkably sombre-coloured birds, 

 and the mimicry cannot be on their side. But there are other 

 brown oiioles, more closely resembling those named, in other 

 Moluccan islands, and yet having no resemblance to the 

 Pliilemon of the same island, as may be seen in the case of the 

 Oriolus p]u€ochrotnus and Philemon gilolensis from Gilolo. Yet 

 the oriole has adopted the same livery which elsewhere is a 

 perfect mimicry. May it not therefore be that we have, in this 

 group of brown orioles, the original type of the family undilfer- 

 entiated? As they spread east and south we may trace the 

 gradation, through the brown striation of the New Guinea 

 bird, to the brighter, green-tinged form of the West Austrahon 

 and the green plumage of the Southern Australian, while west- 

 ward the brilliant yellows of the numerous Indian and African 

 species were developed, and another group, preferring high 

 elevations, passing through the mountain ranges of Java, Suma- 

 tra, and Borneo, intensified the aboriginal brown into black, 

 and hence were evolved the deep reds of the various species 

 which culminate in the crimson of Formosa, Oriolus at Jens, 

 and the still deeper crimsons of O. trailli of the Himalayas. 



It is possible that there may be similarity without mimicry, 

 and, by the five laws of mimicry as laid down by Wallace, very 

 many suggested cases must be eliminated. We all know that 

 it is quite possible to find between species of veiy diffTent 

 genera extraordinary similarity which is not mimetic. Take, 

 lor instance, the remarkable identity of coloration in the case of 

 some of the African species Macronyx and the American S'.ur- 

 fielia, or, again, of some of the African Campophags and the 

 American Agelceus. The outward resemblance occurs in both 

 cases in the red as well as in the yellow-coloured species of all 

 four groups. But we find that xYit Macronyx of Americiand 

 the Cainpophagie of Africa, in acquiring this coloration, have 

 departed widely from the plain colour found in their immediate 

 relatives. If we applied Mr. Scudder's theory on insects, we 

 must imagi'ie that the prototype form has become extinct, Vhile 

 the mimickerhas established its position. This is an hypothesis 

 which is easier to suggest than either to prove or to disprove. 

 Similar cases may frequently be found in botany. The straw- 

 berry is not indigenous in Japan, but in the mountains there I 

 found a potentilla in fruit which absolutely mimicked the Alpine 

 strawberry in the minutest particulars, in its runners, its lil is- 

 soms, and fruit ; but the fruit was simply dry pith, supporting 

 the seeds and retaining its colour without shrinking or falling 

 from the stalk for weeks— a remarkable case, we cinnot say of 

 unconscious mimicry, but of unconscious resemblance. Mimicry 

 in birds is comparatively rare, and still rarer in mammals, 

 which is not surprising when we consider how i-mall is the total 

 number of the mammalia, and even of birds, compared with the 

 countless species of inveilebrates. Out of the vast assemblage 

 of insects, with their varied colours and pattern-, it would le 

 strange if there were not many cases of accidental resemblance. 

 A strict application of Wallace's five laws would, perhaps, if all 

 the circumstances were known, eliminate many accepted in- 

 stances. 



As to cases of edible insects mimicking inedible, Mr. Poulton 

 admits that even unpalatable animals have their special enemies, 

 and that the enemies of palatable animals are not indefinitely 

 numerous. 



