424 NATATORES. 



themselves and their young during the whole of the breeding 

 season. The spring thaw is, in fact, the surface harvest of 

 the Polar Sea, just as the rains of autumn and early winter 

 are the harvest of our estuaries, to which the birds resort, 

 while the frost is sealing up and preserving a store for them 

 against the breeding time. 



The general current produced in the Atlantic by the tides, 

 has, no doubt, some effect upon the distribution of the sur- 

 face water, and consequently upon that of the skimming 

 birds ; but it will generally be found that they nestle where 

 the land stands out to meet the current ; where the cliffs 

 are bold and high, and the water deep, circumstances which 

 usually accompany each other, and which thus have some 

 relation to each other ; but whether that be a relation of 

 co-existence or of cause and effect, there are no complete 

 data for determining, though probability is in favour of that 

 of cause and effect, inasmuch as the sea assails the bold 

 shores, and casts the debris upon the low and flat ones, where 

 the water, from the shelve, has little force, and the sea wind 

 sweeps inland without opposition. 



The natural history of even the surface of the sea is, how- 

 ever, but imperfectly understood. The effect of seasons and 

 tides, that of the variations of atmospheric pressure, of heat 

 and light, and of countless other causes, in themselves little 

 known, are whether in their single results or in the modifi- 

 cation which they produce upon each other, so vaguely com- 

 prehended, that any conclusion drawn from them would be 

 more bold than wise. If, however, human observation should 

 ever be able to bring the subject within the scope of science, 

 the Pelagic birds will form an important point of study, and 

 none of them more so than the storm petrels. 



Of the genus, there are two British species, not found upon 

 the shores, even the rocky shores generally, but rather in 

 those northerly and westerly places, which are more within 



