590 



XATURE 



[October 17, 1895 



modem man of science than by the simple-minded 

 savage or the poet or prophet of aniii|uity." When 

 writing these pregnant words it was not gi\en to this 

 erudite biologist to foresee the revelation of " this Our 

 new law" of dispersal to Charles Dixon, of which the 

 volume under notice is the first proclamation. This great 

 new "law forbids retreat." To Mr. Dixon it has been 

 revealed that the effect of the slow oncoming of a 

 glacial epoch in either hemisphere was not to cause 

 bird-life to retreat in front of the increasing cold, but 

 really to exterminate all those birds having a range of 

 distribution entirely within the refrigerated areas, and 

 to contract the range of such as were migratoiy. Those 

 birds alone survived, therefore, whose former range ex- 

 tended beyond the glaciated areas (the unglaciated por- 

 tions of their range the author calls " refuge areas ") : 

 while all those birds which had no refuge area were 

 totally exterminated, and have since been lost to science. 

 The " law," moreover, forbids species in the northern 

 hemisphere ever to increase their range in a southerly 

 direction, and species in the southern hemisphere ever 

 to increase theirs in a northerly direction ; and only those 

 northern birds or those southern birds whose refuge areas 

 extended on both sides of the equator arc permitted 

 by the " law " to extend their breeding range to regions 

 towards the opposite pole, which presented the most 

 favourable conditions for reproduction. Now "this Our 

 law," we arc told, applies not only to birds, but to all life, 

 and is a universal explanation never thought of by any 

 other "biologist of note," of the migration and geo- 

 graphical distribution of species. To show that this is 

 so, Mr. Dixon applies his law to the distribution of 

 "arctic" types in the flora of the southern hemisphere. 

 .Sir Joseph Hooker long ago explained the presence of 

 the " Scandinavian " element in that flora, by indicating 

 its migration routes along the meridional highlands of 

 the great continental land masses. Hooker, Huxley and 

 Wallace, and doubtless all those other ornithologists and 

 geologists — among whom are .Sharpe and Cieikie — who 

 have, according to Mr. Dixon, gone "beyond their last,'' 

 have been quite misguided by reason of their ignorance 

 of this law. Our latest authority, however, declares with 

 all the emphasis of certainty that " there can have been 

 no emigration of plants from north to south " ; "it could 

 never have taken place " ; Our " law forbids it." The 

 true solution of the question by Mr. Di.xon is, that 

 all the "arctic" plants in the southern as well as in 

 the northern hemisphere, spread from an equatorial 

 centre. Let us take, for example, an "arctic" species 

 common, say, to high northern latitudes, and to New- 

 Zealand, and the Southern .-\ndes or South .Africa. 

 This species must, in the first instance, have arisen 

 in some part of the equatorial regions from a tropical 

 form, by ascending to the cool arctic /ones of one of 

 the mountains— suppose in South America. It must 

 then have followed one of two routes of dispersal. 

 After multiplying it must either have spread right round 

 the equator — the absence of continuous land notwith- 

 standing -crossing again and again the torrid interspaces 

 separating it from other e(|uatorial altitudes, which served 

 it as stepping-stones, till it attained those longitudes 

 whence it could extend its range, as best it might, to its 

 prccnt northern and southern habitats— a migr.ition- 

 NO. 1355, VOL. 52] 



route too remarkable to be easily credited. The alter- 

 nati\e route, so far as regards the southern hemisphere, at 

 all events, would be for the species to spread southwards 

 on one of the continents (say South America), till reach- 

 ing a then-existing Antarctic land, over which it must 

 have gradually dispersed, and in order to reach Smith 

 Africa or New Zealand, it would have to travel north- 

 wards in the very face of Mr. Dixon's inexorable law, 

 which it would thus entirely upset, and with it all the 

 conclusions in the present treatise. How would Mr. 

 Dixon explain, for instance, the distribution of /'ctnca 

 arhorca in South .\nicrica, in West Java, and East 

 Timor ? .Another method of dispersal may perhaps be 

 predicated as possible by some, namely, the independent 

 origin from equatorial ancestors of identical arctic species 

 in high northern and southern latitudes : but any such 

 occurrence is too improbable to be seriously entertained. 

 This law. which seems to us to fail most lamentably to 

 explain the (lispcrs;il of plants, fails not less in regard to 

 the migration of birds. It surely requires no pointing 

 out that during e\ery winter we have numberless boreal 

 species — birds, whales, seals — visiting our shores in 

 retreat south into more genial climes ; the sheep feeding 

 on any high hill, and overtaken at the beginning of winter 

 by storms, hasten for food and shelter to lower levels, 

 where they would continue to remain if there came no 

 moderation in the weather of the uplands ; and our 

 resident redbreasts for the same reason retreat from the 

 woods before the first snow s to the neighbourhood of our 

 homes, and if the winter be specially severe they retreat 

 still further in search of more genial conditions— they do 

 not dare the storm .md liie on the snow. What takes 

 place in miniature during the winter would simply be 

 enacted, there is little doubt, on an extensive scale during 

 a glacial epoch. The migration, to be seen to-day in 

 Western Kuiopc, we arc told by Mr. Dixon, was un- 

 doubtedly initiated with the passing away of the third 

 glacial period, is undertaken expressly for purposes of 

 reproduction, and is "the constant endeavour of what we 

 must now regard as but the relics of such exiled life to 

 regain and repeople the area that it once occupied during 

 pre-glacial lime." Had the migration of pre-glacial 

 times a difliercnt cause or motive than that of to-day ? 

 Why is migration necessary for the purpose-: of breeding .•" 

 Is there not space enough, food enough, antl a better 

 climate in the regions where the migrants winter, ami to 

 which the parents, indeed, return reinforced 1iy their yming, 

 to be dependent on the supplies of that area : How, we 

 may also ask, can the birds which occupied the southern 

 and non-glaciated portion of their range be inspired by 

 "a constant endcaxour to regain" an area their parents 

 had never occupied, and had never e\ en known : for those 

 of their species which had occupied and known the 

 northern part of the range, we are assured rather than 

 retreat a step, chose to die under Dixon's " law. ' 1 lie 

 new Commandment which forbids a southern cxun- 

 sion of breeding area, "renders," according to Mr. 

 Dixon, "a flight south in spring impossible" : and ".ill 

 species do not breed [more grammatically, no species 

 breeds] anywhere smith of their [its] point of enliance." 

 Yet the penguins defy this law, and though southern 

 hemisphere birds, they migrate equator-wards to lirccd. 

 In the spring of this year the present writer witnessed, in 



