1824.] Royal Society. 67 



lungs ; but though swifts and martins, it is observed, in reply 

 to this conjecture, frequently splash in the water over which 

 they are skimming, yet they never immerge themselves in it, and 

 indeed if they were to do so, their wings would become so wet 

 as to prevent their flying. The common duck, when pursued 

 and forced to dive repeatedly, by a water-dog, arrives at the 

 surface again much exhausted ; as is likewise the case with 

 grebes and auks, after repeated diving. Dr. Jenner had been 

 in the habit of receiving Newfoundland dogs from that country, 

 and had ascertained that they never continued under water for 

 more than thirty seconds, and even then seemed confused when 

 they came up. It had been asserted that negro and other 

 divers remained under water several minutes ; but Dr. J. con- 

 ceives this assertion to be grounded only on a vague guess, 

 and that the time was not measured by a stop-watch. 



The next division of the paper relates to the remarkable 

 effect of instinct in birds, of their returning to build on the 

 same spot for many successive seasons. The author took twelve 

 swifts from their nests in a barn, indelibly marked them all, by 

 taking off two claws from one foot of each, and then set them 

 at liberty. Some of them were caught again on the same spot, 

 at the expiration of a year, and others after two years had 

 elapsed ; they were not attended to afterwards, but at the expi- 

 ration of seven years from their original capture, one of these 

 marked swifts was brought in by a cat. 



Dr. Jenner next proceeds to state, as the cause of the migration 

 of birds, that the tumid and enlarged state of the testes in the 

 male, and of the ovariain the female, at the season of their depar- 

 ture, prompt the animals to seek those countries where they can 

 obtain proper succours for their offspring; — that, in fact, the 

 nestlings are the objects of this provision. The parent birds 

 leave the countries they migrate from at a time when their own 

 wants are completely supplied ; and they remain in those to which 

 they migrate, no longer than suffices for the rearing - of theiryoung. 

 Thus the swifts arrive in this country about the 5th or o'th of 

 April, and depart hence about the 10th of August. — Dr. Jenner 

 here observes, as a remarkable circumstance, that Ray, who 

 attributed the migration of fishes to its true cause, that of 

 seeking proper situations for spawning, overlooked the cor- 

 responding impulse as actuating birds. — The martins leave 

 this country successively, some continuing to rear a brood 

 ,h li ter than others : many of these birds roost in the walls 

 of Berkeley Castle ; and Dr. Jenner found, by dissecting a num- 

 taken at the same time, that the ovaria of the females were 

 iu a variety oi slates ; in some the eggs being no bigger than 

 hemp seed, while in others they were as large as peas ; the 

 testes of the males exhibited analogous degrees of tumidity. 



Swallows are seen flying over pools and waters iu spring, 

 in search of the gnats on which they are then obliged to 



f2 



