434 



NATURE 



{March 13, 1879 



tinent, at the present time revealed to us by shallow 

 soundings and some mountain-peaks changed into islands, 

 may be indubitable. I, at least, in regard to Europe and 

 Africa do not question it, and it may be true all the world 

 over. But this same hypothesis has been more or less 

 hinted, if not absolutely promulgated, by Prof. Baird 

 {American fournal of Science and Arts, May, 1866) and 

 Capt Hutton {Trans. N. Zealand Inst., v. p. 235). So far, 

 then, there is nothing novel in the Doctor's views. 



In like manner Dr. Weissmann seems to me to have 

 been anticipated by Capt. Hutton {jit suprci) and by Mr. 

 Wallace (NATURE, vol. x. p. 459) in his explanation of 

 v/hy birds migrate at all. The only material difference 

 between the last of these authors and the Doctor is that, 

 ■while Mr. Wallace most rightly (as it seems to me) regards 

 migration as originating with the bird at its breeding- 

 quarters, Dr. Weissmann considers it to begin with the 

 bird in its winter retreat. Perhaps this does not much 

 matter, but it is as well not to put the cart before the 

 horse if you want to prosper in your journey, and so long 

 as lack of food be admittedly the strongest incentive to 

 migration, it seems preferable to look on migration as be- 

 ginning where that incentive is strongest. This, it 

 scarcely needs to be said, is when, towards the close of 

 summer, the supply of food grows scarce. 



However, the most important part of the whole busi- 

 ness is the question how the birds find their way to the 

 places whither they repair, whether for the purpose of 

 breeding or for that of procuring sufficient sustenance. Dr. 

 Palmdn regards it simply as a matter of " experience," 

 and Dr. Weissijiann hardly differs from him. It is 

 "practice" {Uebttng), says the latter — not indeed the 

 practice acquired by the single bird, but the practice 

 acquired by the whole species. " This faculty ( Virtuositdt) 

 of finding the way has not arisen suddenly, but most 

 gradually, in the course of many thousands of generations." 

 Now with all my faith in the marvellous results which 

 are doubtless produced by the hereditary transmission of 

 certain qualities, I think some caution is needed before 

 we accept "practice " as the true explanation of the 

 puzzle. Dr. Weissmann says that he does not see what 

 more is needed than a fine power of observation and a 

 keen eye to take in every thing of importance for a know- 

 ledge of the way, and then a very remarkable memory for 

 places by means of which all details of the long route 

 shall be retained. The knowledge of direction {Orieniir- 

 ung) will then follow of course. Subsequently he takes 

 the instance of a woodpecker being able to find the tree 

 containing its nest, though surrounded by hundreds of 

 similar trees, and declares that this knowledge or sense of 

 direction must, in the case of birds when migrating, be 

 wholly analogous. He suggests also that the height at 

 which birds sometimes fly, referring apparently to a remark 

 by Mr. Tennant (Naturi:, xiii. p. 447), would enable them 

 to cross the Mediterranean, and seldom or never lose sight 

 of land. 



This may be ; but migration goes on in other parts of 

 the world, and a good explanation ought to apply else- 

 where. Will Dr. Weissmann's' hold good for our Anti- 

 podes? In regard to New Zealand Capt. Hutton has 

 remarked {Trans. N.Z. Inst., v. p. 235) :— " That we should 

 have two cuckoos which migrate regularly to other 

 countries, each more than a thousand miles distant, is a 

 fact that deserves special attention, for I know of no 

 parallel case in any other part of the world, the distance 

 across the Meditei-ranean being less than half that tra- 

 velled over by our summervisitors." These two cuckoos are 

 Chrysococi-yx liuidus and Eudynamis taitensis, the former, 

 it is supposed, making its annual journeys to and from 

 Australia,' and the latter to and from the Friendly Islands 

 or the Fijis, it being found in both groups, to say nothing 

 of other places further off. Let us consider the case of 

 the Endynaniis. Due north of New Zealand there appears 



t ' I should rather suspect to and from New Caledonia. 



to be no land until Fiji is reached, but a little to the 

 westward of the direct line lies Norfolk Island and its 

 companions, and about as far to the eastward are 

 the Kermadecs. Of these, the most southerly ij 

 450 miles from New Zealand, and the most northerly 

 about the same distance from Pylstaart, an outlier of the 

 Friendly Islands. The Eiidynamis starting from New 

 Zealand for the northward would have nothing to supple- 

 ment its inherited "sense of direction" save the land- 

 marks offered to the right and left by the Kermadecs and 

 Norfolk Island respectively. To see the former it would 

 have to mount to the height of some twenty miles,^ and 

 to again mount about as high on leaving the Kermadecs 

 on its way to Pylstaart. It might be urged that the bird 

 having by "practice" a sense of the direction in which 

 it ought to go, might fly half the distance, keeping the 

 land of departure in sight — though, considering the 

 position of birds' eyes, this would not be easy — and then, 

 without exceeding the level of six or seven miles, it 

 might behold the Kermadecs, but even this is an eleva- 

 tion far beyond Dr. Weissman's 20,000 feet. The route 

 by Norfolk Island being longer, need not here be 

 discussed. 



It is much to be desired that something positive were 

 known as to the height at which it may be possible for 

 birds to perform their passages, but on this point we have 

 (so far as I am aware) little information. The experi- 

 ments made by Mr. Glaisher on the six pigeons taken up 

 in his celebrated balloon ascent, September 5, 1862 

 ("Rep. Brit. Ass.," 1862, p. 385), unfortunately admit of 

 no definite deductions. One pigeon thrown out at the 

 height of three miles " extended its wings and dropped . 

 as a piece of paper." A second at four miles "flew 

 vigorously round and round, apparently taking a 

 dip each time." A third between four and five miles 

 " fell downwards as a stone." A fourth at four miles, 

 in the descent, " flew in a circle," and then alighted 

 on the balloon. The two remaining pigeons were 

 brought down, and one was found to be dead ! Perhaps 

 a little more "practice" or " experience " was wantedj 

 but at any rate the results do not seem to favour the 

 notion that birds can fly comfortably at those heights.' 

 Nor is this surprising, considering the well-known effect^ 

 of the rarification of the air at great heights. I of course 

 pretend to no special knowledge of this subject, but Mr^ 

 J. W. L. Glaisher, F.R.S., kindly informs me that at ai 

 elevation of five miles the density of the air is abou| 



— of what it is on the earth's surface, at an elevation 



27 



seven miles about --, and often miles about -^ . Ikno\ 



41 7-5 



not whether experiments have been made to test the ^n\ 

 durance of a bird's life under such a condition as the last 

 but it could of course be easily produced under an air| 

 pump. It would not be so easy to test the power 

 flight under the same condition. It is only obvious ths 

 the power would be very greatly diminished, and I should 

 be glad to learn the results of any investigation of thil 

 kind. Physicists and physiologists might here giv| 

 ornithologists great help. 



But to return to the question of distance and sigh^ 

 How comes it that the American Golden Plover {Chart 

 drius virginicus) passes regularly every year in larg 

 flocks over the Bermudas, 600 miles from the neares 

 point of land, and that a point whence these flocks cer^ 

 tainly do not take their departure. If the islands are^ 

 "still vext" by stormy weather, the flocks alight and 

 afford the inhabitants a good deal of sport. If the 

 weather be fine, the flocks seem to continue their south- 

 ward course. Nor is this plover the only regular visitant. 

 The American Night-Hawk {Chordiles popetue) is as con- 

 stant in its appearance at spring and fall, and so are 



' Of course the exact height would depend on the elevatioo of the land, 

 concerainif which I have no information. 



