September 21, 1893] 



NATURE 



493 



ihtir longer-billed relations. In the breeding season, therefore, 

 <hey would have fewer eggs and a weaker progeny. Often, as 

 we know, a weakly bird will abstain from matrimony alto- 

 gether. The natural result of these causes would be that in 

 toune of time the longest-billed variety would steadily predomi- 

 nate over the shorter, and, in a few centuries, they would be the 

 sole existing race ; their shorier-billed fellows dying out until 

 that race was extinct. The converse will still hold good of the 

 SI cut-billed and weaker billed vaiieties in a rocky dislrict. 



"Here are only two causes enumerated which miyht serve to 

 cr«alf , as it were, a new species from an old one. Yet they are 

 perfectly natural causes, and such as I think must have 

 occurred, ami .ire possibly occurring still. We know so very 

 little of the caues which, in the majority of cases, make species 

 laie or common that there may be hundreds of others at work, 

 some even more powerful than these, which go to perpetuate 

 and eliminate certain forms 'according to natural means of 

 selection. ' " 



It wou'd appear that those species in continental are.as are 

 equally liable to variation with tho.5e which are isolated in 

 limited areas, yet that there ate many counteracting influences 

 which operate to check this tendency. It is o'ten assumed, 

 where we fii.d closely allied species apparently inter-breeding 

 at the centre of their area, that the blending o( forms is caused 

 by the two races commingling. Judging from insular experience 

 I should be inclined to believe that the theory of inter-breeding 

 is beginning at the wrong end, but rather that while the general- 

 ised iorms remain in the centre of distrihuion, we find the more 

 decidedly distinct species at the extremes of the range, caused 

 not by inler-hrceding, but by differentiation. To illustrate this 

 by the group of the bli!e titmouse. We find in Central Russia, 

 in the centre of distribution of the family, the most generalised 

 iorm, J'ariis phs/ai, partaking of the characters of the various 

 species east, west, and south. In the north-east and north it 

 becomes differentiated as /'. cyancus \ to the soulh-we>t and 

 south into P. iicritlcits and its various sub-species, while a 

 branch extending due east has assumed the form of Fflavi- 

 falus, bearing traces of aflSnity to its neisjhbour P. cyaveus 

 in the north, which seems evidently to have been derived 

 from it. 



But the scope of field observation does not cease with geo- 

 2r>pi^>cal distribution and modification of forn>. The do et 

 systematist is very apt to overlook or to take no count of 

 habits, voice, modification, and other features of life which 

 have an important hearing on the modification of species. To 

 take one instance, the short-toed lark {Calatidrella bi-achydac- 

 lyla) is spread over the countries bordering on the Mecliier- 

 ntnean ; but, along with ir, in Andalusia alone is found another 

 species, CaL bu^tida, of a ratherriarkercolour,and with the secon- 

 daries generally somewhat shorter. Without further knowledge 

 than that obtained from a comparison of skins, it might be put 

 down as an accidental variety. But the field naturalist soon 

 recognises it as a most distinct specie.s. It has a different voice, 

 a differently-shaped nest ; and, while the common species breeds 

 in the plain?, this one always resorts to the hills. The Spanish 

 shepherdson the spot recognise theirdistinctness,and haveaname 

 for each species. Take, again, the eastern form of the common 

 song-thrush. The bird of K'orth China, Turdus auriliis, closely 

 resembles our familiar sp-cies, but is slightly larger, and there 

 is a minute difference in the wing formula. But the fi^ld natu- 

 ralist has ascertained that it lays eggs like those of the missel- 

 thrush, and it is the only species closely allied to our bird which 

 does not lay eggs of a blue ground colour. The hedge accentor 

 of Japan (Acctnlor rubidus) is distinguished from our most 

 familiar friend, Accoilor mcdtilaris, by delicate differences of 

 hue. But, though in gait and manner it closely resembles it, I 

 was surprised to find the Japanese bird strikingly distinct in 

 habits and life, being found only in forest and brushwood 

 seveial thousand feet above the sea. I met with it first at Chin- 

 KDze — 60CO feet — before the snow had left the ground, and in 

 svinmer it goes higher still, but never descends to the cultivated 

 land. If both species are derived, as seems probable, from 

 Acctntor immacu/alus of the Himal.iyas, then the contrast in 

 habits is easily explained. The lofty mountain ranges of Japan 

 have enabled the settlers there to retain their original habits, 

 for which our humbler elevations have afforded no scope. 



On the solution of the problem of the migration of birds, the 

 most remarkable of all the phenomena of animal life, much less 

 aid has been contributed by the observations of field naturalists 

 than might reasonably have been expected. The facts of migra- 



tion have, of course, been recognised from the earliest limes, 

 and have affoided a ihtme for Hebrew and Greek poets 30CO 

 yeais ago. Theories which wculd explain it are rife enough, 

 but it is only of late jears that any systematic effort has been 

 made to classify and summarise the thousands of data and notes 

 which are net ded in order to draw any satisfactory conclusion. 

 The observable facts may be classified as to their bearing on the 

 whither, when, and how, of migration, and after this we may 

 possibly arrive at a true answer to the Why? Observation has 

 sufficietttly answered the first question. Whither ? 



There are scarcely any feathered denizens of earth or sea to 

 the summer and winter ranges of which we cannot now point. 

 Of almost all the birds of the holo-arctic fauna, we have ascet- 

 tained the breeding-places and the winter resorts. Now that 

 the knot ard the sanderling have beer, successfully pursued even 

 to Grinneil Land, there remains but the cuilew sandpiper 

 {Triiiga iubarquata), of all the known European birds, whose 

 breeding ground is a virgin soil, to be trodden, let us hope, in a 

 successlul exploration by Nansen, on one side or other of the 

 North Pole. Equally clearly ascertained are the winter 

 quavlers of all the migrants. The most casual observer cannot 

 fail to notice in any part of Africa, north or south, west coast 

 or interior, the myriads of familiar species which winter there. 

 As to the time of migration, the earliest notes of field naturalists 

 have been the records of the dates of arrival of the feathered 

 visitors. We possess them for some localities, as for Norfo.k 

 by the Marsham family, so far back as 1736. In recent years 

 these observations have been cai-ried out on a larger and more 

 systematic scale by Middendorff, who, t'orty years ago, devoted 

 him-elf to the study of the lines of migration in the Russian 

 Empire, tracing what he called the iiopipteses, the lines of 

 simultaneous arrival of particular species, and by Prof. Palmen, 

 of Finland, who, twenty years later, pursued a similar course of 

 investigation; and by Prof. Baird on the migration of North 

 American birds ; and subsequently by Severtzoff as regards 

 Central Asia, and Menzbier as regards Eastern Europe. As 

 respects our own coasts, a vast mass of statistics has been col- 

 lected by the labours of the Migration Committee appointtd by 

 the British Association in 1880, for which our thanks are due 

 to the indefatigable zeal of Mr. John Coideaux and his colleaj^ue 

 Mr. lohn Ilarvie Brown, the originators of the scheme by which 

 the lighthouses were for nine years u-ed as posts of observation 

 on migration. The reports of that committee are familiar to 

 us, but the inferences are not yet worked out. I cannot but 

 regret that the committee has been allowed to drop. Prof. 

 W. W. Cooke has been carrying on similar ob>ervations in the 

 Mississippi valley, and others, too numeious to mention, have 

 done the same elsewhere. But, as Prof. Newton has truly 

 said, all these efforts may be said to pale before the stupendous 

 amount of information amassed during more than fifty years by 

 the venerable Herr Giitke, of Heligoland, whose work we 

 earnestly desire may soon appear in an English version. 



We have, through the labours of the wiiter- I have named, 

 and many others, arrived at a fair knowledge of the When? of 

 migration. Of the How ? we have ascertained a little, but very 

 little. The lines of migration vary widely in different specit.s, 

 and in different longitudes. The theory of migration being 

 directed towards the magnetic pole, first started by Middendorff, 

 seems to be refuted by Baiid, who has shown that in North 

 America the theory will not hold. Yet, in some instance!!, 

 there is evidently a converging tendency in nnrihward migra- 

 tions. 'I he line, according to Middendorff, in Middle Siberia is 

 due north, in Eastern Siberia south-east 10 north- west, and in Wes- 

 tern Siberia from south-west to north-east. In European Russia 

 Menzbier traces four northward routes : (l) A coast line coming 

 up from Norway round the North Cape to Nova Zembla. (2) 

 The Baltic line with bifurcation, one jiioceeding by the Gulf of 

 Bothnia, and the other by the Gulf of Finland, which is after- 

 wards again sulidivided. (3) A Black Sea line, reaching 

 nearly as far north as the valley of the Pclchora ; and (4) the Cas- 

 pian line, passing up the Voign, and reaching as far east as the 

 valley of the Obi by other anastomosing streams. 



Palmen has endeavoured to Irace the -lines of migration on the 

 return autumnal journey in the eastern hemisphere, and has 

 arranged them in nine routes : (i) From Nova Zembla, round 

 the Westof Norway, to the Biiiish Isles. (2) From Spitzbergen, 

 by Norway, lo Britain, France, Portugal, and West Africa. 

 (3) From North Russia, by the Gulf of Finland, Ilulstein, and 

 Holland, and then bifurcating to the West Coast of France on the 

 one side, and on the other up the Rhine to Italy and North 



NO I1'47. VOL. 48] 



