16 DUBLIN NATURAL niSTORY SOCIETY. 



witliin the extreme limits of its standard of existence, or even in a dis- 

 trict which, whilst falling within the range of its existence, is yet 

 excluded from its standard. That summer and winter migrations take 

 place almost always, if not always, in a line due north and south, is an 

 argument in favour of this explanation. I cannot call to mind a case 

 in which uidificatory migration is markedly east and west over an 

 extended area. In some few cases, it is true, the line of migration is 

 not bounded by strictly parallel longitudinal lines east and west ; but 

 this might have been premised, since neither can areas similar in their 

 climates be bounded by parallel lines ; but in tracing such areas north- 

 wards or southwards, we meet with divergent outlying sub-areas which 

 copy the climates northward or southward of them rather than those 

 adjacent to them in a direct line. Such exceptions are, however, rarer 

 than might have been expected. 



A few instances selected from among the birds of the British Isles, 

 as these have been best studied, and, of these, choosing species which, 

 occurring north of Great Britain, might have been expected to occur in 

 Ireland also, but do not, will suffice to illustrate this. 



The nightingale (P. luscinia) occurs in Cumberland (Carlisle), Lan- 

 cashire, Yorkshire, Lincolnshire, JSTorfolk, Suffolk, Essex, Middlesex, 

 Kent, Somerset, Hants, Sussex, and East Devon. It is common only in 

 the easternmost of these shires, being rare in Cumberland, Lancashire, 

 Somerset, and East Devon, unknown in West Devon and Cornwall, 

 apparently so in Scotland, certainly in Wales. IN'orth of Britain it 

 occurs in Sweden, Eussia, and Siberia; and south in the eastern parts 

 of France, Germany, and Spain, wintering in IS'orth Africa, Egypt, and 

 Syria; unknown in the Channel Islands and Brittany. JN'ow if these 

 points be connected on the map, it will be seen that the regular line of 

 migration is to the eastAvards of a line which cuts off Ireland, Wales, 

 Scotland, and part of England. 



The red-backed shrike {Lanius collurio) occurs in Cumberland, Wales, 

 and the south of England ; not in Scotland ; north, in Eussia, Sweden, 

 Denmark ; south, in Germany, Erance, Spain, North Africa, and Cape 

 of Good Hope,— its line of migration being slightly to the westward of 

 that of the nightingale, but still excluding Ireland. 



The pied fly-catcher {^M. luctiiosa), in like manner, whilst extending 

 as far north as Scandinavia and Central Eussia, is rare in the south- 

 western shires, and unknown in the north-western. 



The lesser white-thi'oat (C. sylviella) extends into Wales, but is rare 

 there, showing that this is an outlying station. It is unnecessary to 

 multiply examples further, for the same laws will be found to be more 

 or less strictly applicable to all the regular summer emigrants of Great 

 Britain which are unknown as such in Ireland. 



Taking such a^aewof the case, migration resolves itself into this : — 

 A species (the white-fronted goose, Anser alhifrons, for instance) rears 

 its young in the north during the summer season of that region, when 

 food, of the kind suitable for those young, is easily procurable. After 

 the young are fully developed, winter sets in, and either destroys that 



