F.B.KIRKM AN: VARIATION IN NESTS OFTERNS. 107 



it that those individuals of the species which hne their 

 nests differ in their choice of material ? Let us note that 

 this choice is limited by two conditions, the most obvious 

 being the accessibility of the material. But though 

 accessibility limits choice, it does not necessarily determine 

 its direction. The material of the Terns' nest above 

 described was equally accessible to all. A more striking 

 illustration is, however, provided by the following details 

 of the material used in six Thrushes' nests, all built in 

 gorse bushes within fifty yards of one another, one only 

 being old : (1) gorse ; (2) an old nest of grass, with a 

 small amount of gorse and twigs, and one bit of wool ; 

 (3) grass, moss, stalks ; (4) grass, moss, bracken ; (5) grass, 

 a little wool ; (6) grass, moss, hairs. Obviously, the most 

 accessible material for all was gorse, yet this was utilised 

 by not more than two. 



The choice of materials in the Terns' nests described 

 appears to have been limited only by accessibility, but 

 in the vast majority of cases, if not in all, there is a 

 second quite distinct limiting condition which may briefly 

 be called the law of species. For instance, the Thrush, 

 though using a wide variety of materials in the normal 

 construction of the outside of its nest, appears to be 

 debarred from using twigs only. The material of the 

 inside lining is subjected to still greater restrictions, 

 being confined, " freak " nests apart, to dung or mud 

 studded with bits of rotten wood. This limitation is, 

 no doubt, to be explained, as Mr. Alfred Russel Wallace 

 has pointed out, by some peculiarity in the structure or 

 habits, past or present, of the species. It would be 

 interesting to know what this is in the case of the Thrush. 

 What led it to scorn the Blackbird's addition of grass, 

 or the wool, hair, feathers, used by other species ? Put 

 more generally, the question is one of the origin of 

 specific as distinct from individual variations. 



Within the limits thus imposed upon it by accessibility 

 and the law of the species, why does one individual select, 

 say, twigs, another pebbles ? It may be that the young 



