DUBLIN NATURAL HISTORY SOCIETY. 15 



surprise that scientific men should have, long ago, made its phenomena 

 a subject of study, and have traced many of its laws. There are, how- 

 ever, certain anomalies in distribution as to the occurrence of species in 

 certain countries at regular intervals, which, in closely adjoining ones, 

 are regularly migratory. These, it appears, have not attracted as much 

 attention as they deserve, and are connected with migration. The most 

 remarkable is that to which the title of this paper refers, viz., the oc- 

 currence of summer migrants in winter. Before entering on this sub- 

 ject, however, it will be necessary to lay down briefly what is here 

 understood by migration in birds. 



In a former communication of mine, brought before this Society in 

 March, 1858,^-' when treating on the distribution of ferns in Ireland, 

 three general laws were enunciated as governing the distribution of 

 organized species. 



These were — That all species require a certain fixed standard amount 

 of the great physical agents for their due development. 



That this standard may be altered within certain limits, without 

 destruction of the species, though at the expense of its well-being {range 

 of existence). 



And last, that there are certain fixed limits to this range, outside of 

 which the species must absolutely perish {limit of existence). 



Now, remembering that the standard of existence is not necessarily 

 uniform in different species, nor even at difi'erent ages of the same 

 species, the standard of existence in the adult birds having a more 

 extensive range than in the young ; and bearing in mind that the food 

 of the two difi'ers greatly in quantity : periodic migrations — that is, 

 those strongly marked passages, at a fixed period of the year, of species 

 from one area to another — are easily explained ; excluding at present 

 those migrations in which the passage performed is merely the shifting 

 from one district to another of a similar area, necessitated by the failure 

 of food. This explanation is not contradicted by the occasional resi- 

 dence through the entire j'ear, or breeding of individuals in districts 

 intermediate between the actual northern and southern hiemal and 

 SBstival residences of the species ; because it must be remembered that 

 the limits of the standard of existence of a species are sometimes very 

 extended, occasionally almost equally so, with the limits of the range of 

 its existence ; also, that the climate of every part of the districts passed 

 through in migration are not either uniform or uniformly constant in 

 each year, and hence that it may occur that a bird in its northerly pro- 

 gress, from some cause or other, late in its migration — as, for instance, 

 a weakling left behind at the annual starting-point whence all the 

 stronger birds pushed boldly forwards for the north — may, on finding 

 the instinct of nidification too strong upon it, build its nest in the first 

 spot which copies its proper summer region sufiiciently nearly to fall 



* " On the Distribution of Ferns in Ireland, with a List of some of the most remark- 

 able Localities in which they occur."— Proc. Nat. Hist. Soc, vol. ii., p. 91. 



