17c 



NA TV RE 



[December 22, 1892 



the rest of the species by the lofty mouniain ranges, the 

 deserts, and other physical barriers, which would effectu- 

 ally assist a forest or woodland haunting species. More 

 and more severe became the winters, longer and longer ; 

 the glaciers descended lower and lower, exterminating 

 or driving before them all living things. At last the 

 Spotted Flycatcher, or the form which then represented 

 this species, came to be divided into two enormous 

 colonies— an African one and a Chinese one— the indi- 

 viduals of each being completely isolated from each other, 

 summer and winter alike. During the ages that this 

 state of things continued, the Flycatchers became 

 segregated into two species, owing primarily to the 

 absence of any intermarriage ; the eastern race became 

 smaller, the tail shorter, and the breast-streaks broader ; 

 or the western race became larger, with a longer tail and 

 narrow breast-streaks. It is almost impossible to say 

 which form now most closely resembles the ancestral 

 species ; but such are the present differences between 

 the two races known to ornithologists respectively as 

 Muscicapa grisola (the Western and British form) and 

 Muscicapa griseisticta (the Eastern form). Such was 

 the state of things at the close of this Inter-Glacial 

 Period. 



" Then came the gradual immigration north again, as 

 precession and lower eccentricity initiated a milder 

 climate. Age after age the journey in the spring became 

 longer. Certain routes to and fro came to be recognized 

 highways of passage ; and so imperceptibly did the 

 northern breeding grounds expand that the birds became 

 regular migrants, looking upon the movement north to 

 higher and cooler latitudes each spring as an undertaking 

 never to be missed. Warmer and warmer became the 

 southern haunts, stimulating and widening migration 

 flight to the cooler temperatures prevailing near the 

 edges of the retreating glaciers, where a suitable breeding 

 climate could only be found. 



" Let us confine our attention solely to the birds that 

 bred in the British Islands, In the Prs-Glacial ages 

 this area formed part of Continental Europe ; a rich and 

 fertile corner, abounding in insect life, full of haunts the 

 Flycatcher loved. After the banishment of its race 

 and the exile of its ancestors in Africa, the northern 

 journey at first did not extend further than the edges of 

 the glaciers on the Mediterranean coasts of Europe. But 

 as these disappeared, and a warmer climate began to 

 prevail in higher latitudes, the annual summer flight was 

 mcreased. Every century the northern breeding range 

 had increased, creeping slowly across France ; higher and 

 higher with the growing vegetation ; nearer and nearer to 

 the haunts of old. During the slow, gradual elevation 

 and submergence that isolated Albion from the rest of 

 Europe during Post-Glacial time, the regular spring 

 journey across the sea became wider and wider ; but with 

 the intense and inherited love of home in their tiny 

 breasts, the individuals that were born and bred in this 

 district never failed to return each year. For 60,000 years 

 or more has this species now crossed the sea, returning 

 every season, not only to our islands, but each pair of 

 individuals, as long as they live, come back to the exact 

 locality of their previous nests. This long journey, 

 gradually growing longer and longer during thousands of 

 years, until it is now at least a thousand miles in length, 

 has grown to be a deeply-rooted custom sanctioned by 

 the practice of ages of experience and need, and looked 

 upon now as part of the Flycatcher's very existence ! " 

 (pp. 58 62), 



This, we think, is Mr. Dixon at his best, and we are 

 anxious that our readers should so see him. He goes on 

 to call it a " thoroughly demonstrable instance," which 

 shows what his idea of a demonstration is. We do not 

 deny that all may have happened as he here prescribes, 

 NO. 1208, VOL. 47] 



but who knows that it did 'i To begin with, we may ask 

 what proof is there of the existence on the earth itself of 

 Muscicapa grisola "when the Sub-Polar regions of the 

 northern hemisphere last enjoyed a warm, almost semi- 

 tropical climate " ? That its ancestors then lived we do 

 not doubt, but who can tell us what they were like? 

 What is meant by its "becoming partially nocturnal 

 during the Polar night " ? If so its eyes must since have 

 undergone a considerable change, and that would hardly 

 be unattended by a corresponding change in other parts of 

 the bird's structure. But still it is a pleasing suggestion 

 that "its conditions were easy" in those millenniums, 

 and we hope Mr. Dixon may be right, though for our own 

 part we cannot help fearing that the struggle for existence 

 must have already begun. Certainly it set in at last, and 

 those terrible glaciers drove the poor bird before them, 

 with the effect — Mr. Dixon, we think, is to blame for not 

 giving us the geographical details (which of course must 

 be known to him) of the process — of dividing the species 

 or the form which represented it, and may be presumed 

 (though this is not mentioned) to have by that time got 

 rid of its owls' eyes, " into two enormous colonies — an 

 African one and a Chinese one." These were so isolated 

 that mter-marriage between the individuals of the two 

 portions was impossible, the remarkable consequence of 

 which was that " the Eastern race became smaller " than 

 the Western — a character distinctive indeed of the human 

 races, the Pygmies excepted, now inhabiting the same 

 lands — but with " the tail shorter " — a contradictory 

 character, since the long tail of a Celestial is the really 

 important part of him. We are also told that " it is 

 almost impossible to say which form now most closely 

 resembles the ancestral species," an unexpected confession 

 of ignorance (the " almost" is good) after so much infor- 

 mation, but one to which we see the necessity of bowing. 

 However, what is the upshot of all this ? And how is any 

 " law " illustrated by it .? Setting aside the vagaries on 

 which we have just commented, it reads to us as being 

 merely an amplification of suggestions that were tenta- 

 tively and cautiously submitted in these columns more 

 than eighteen years ago (N ature, vol. x. pp. 416 and 459). 

 The partiality of birds for their old homes was then, and 

 (so far as we know) for the first time, pointed out as one 

 possible factor in establishing migratory habit; and, as 

 another (and equally for the first time), the growing diver- 

 gence of breeding and feedmg areas through chmatic 

 causes was briefly and clearly set forth by Mr. Wallace. 

 Notwithstanding Mr. Dixon's assertions, he does not seem 

 to have advanced the question one bit, but he has over- 

 whelmed it in a flow of words with a great deal that is, 

 and apparently always will be, incapable of proof. Here 

 and elsewhere throughout this volume we are brought to 

 face one of that school of biologists, the growth of later 

 years, which may be called the Assertive. In some 

 respects it is a very nice one to join. You have only got 

 to say what first comes into your head, and all 

 goes well. Everybody that differs from you is a fool. To 

 some extent this school resembles that Dogmatic one 

 which a few naturalists here and there still rcmemb.-r, 

 inasmuch as the dissentient from either was regarded 

 with the same contempt. The Dogmatists have had their 

 day, but if we look back upon their doings, we shall see 

 that in most cases they had something to go upon which 



