268 



BIRDS. 



No. 122.— Eccleshall, two, 15° W. long. Two shot out of about 

 twenty, 22nd May (Mr. S. Yates, Times, 28tli May 1863). 



No. 124. — Six shot on the links about the first week in June 

 1863 (Edinburgh Evening Courant, 13th June 1863; Col. 

 Drummond Hay, in lit., 15th June 1863). 



Eegarding the causes of this phenomenal invasion, I will simply 

 refer to what has been written and previously epitomised by Pro- 

 fessor Newton in the above-mentioned paper, and say that I implicitly 

 accept the deductions he himself expresses, viz. that it "resulted 

 from ordinary increase." Indeed, I may even perhaps make bold 

 to say that it is an illustration of a great natural law, applying to 

 many other irruptions of species, though not of all. In fact, it may 

 well be what I have attempted to express by the phrase "congestion 

 at previous centres," and "finding the direction of least resistance." 



Exceptions are found in the irruption of such a species as the Little 

 Auk, caused by local and sudden gales of wind, or circumstances close 

 to the normal winter quarters of the birds' distribution. But even in 

 this case, the Little Auks found the "way of least resistance," as, 

 I think, may be gathered from the article by Mr. W. Eagle Clarke.^ 



I think that the exposition of this "natural law," if law it be, 

 and which I, at least, look upon as a very powerful cause of these 

 initiatory movements of many birds on migration, requires no further 

 treatment at my hands in this place. Pages 219-22 of Professor 

 Newton's paper appear to me to carry conviction. 



Much, however, remains to be observed and proved as regards 

 these so-called "lines of least resistance," when they come to be 

 applied to many different species ; because a selected route by any 

 one species may not be that selected by another. In other words, 

 what may be the " line of least resistance " to one bird, may be to 

 another the "line of greatest resistance." And this may well be 

 found to apply not only to a species, but also even to individuals of 

 the same species. (I eliminate from this statement the possibilities 

 connected with sub-species. That opens out abundance of study for 

 the future.) 



Passing now to the second and greater invasion, I first " take up 

 the tale as told to me," in the material collected, and in the digest 

 of it by the late Eev. H. A. Macpherson, the original documents 

 being still in my possession.- And, at the same time, to call 



1 Annals Scot. Nat. Hist., 1895. 



2 The Visitation of Pallas' s Sand-Grouse to Scotland in 1888, etc. (pamphlet): (Porter, 

 18 Princes Street, Cavendish Square, London, 1889). 



