58 Dr. Jenner on the Migration of Birds. 



one of those incidents in natural history that not only baffles 

 our searches, but almost eludes our guesses ! " Thus Mr. 

 White. 



Now, should the principle I have laid down be admitted, 

 namely, that these birds come here for scarcely any other pur- 

 pose than to produce an offspring, and retreat when the task 

 ts finished, how easily will all circumstances be reconciled ! 

 and how little mysterious will those things appear which na- 

 turally seemed unaccountable, not only to the amiable author 

 from whom the foregoing passage is taken, but also to others 

 who have written before on the same subject. 



It is somewhat remarkable that so sagacious a philosopher 

 as the illustrious and learned Ray, who so clearly saw the 

 object of migration in fishes, should not also have been led 

 to a sight of it in birds. After making a very just observation 

 respecting salmons, that quit the sea and ascend up rivers 

 with no other view than to find a place of security for their 

 spawn in the sand, he directly says again, adverting to birds, 

 " What moves them to shift their quarters? You will say the 

 disagreeableness of the air to the constitution of their bodies, 

 or want of food *." 



The spring migrating birds do not arrive here at first in 

 very large numbers. It may be observed, that in the early 

 part of April a few swallows may be seen ; soon after these a 

 few solitary martins, and as the month advances now and then 

 a swift. On the walls of Berkeley Castle martins build their 

 nests in great numbers. I availed myself of their situation, 

 and took several of them on the same night, the latter end of 

 May. On dissection, the cause of their gradual and succes- 

 sive migrations appeared obvious, the testes and ovaria being 

 in very different states of progressive forwardness. While 

 one bird presented embryo eggs in the ovarium as large as 

 peas, in another they were found no larger than hemp-seed. 

 These were the extremes; for in the other birds there ap- 

 peared all the intermediate stages, from the enlargement of 

 the ovaria, sufficient to give the stimulus for migration, to the 

 degree of forwardness just described. The same gradations 

 in the state of the testes of the male corresponded with that of 

 the ovaria in the females. This progressive arrival is not con- 

 fined to the swallow tribe : all the birds that come early in 

 the spring appear in the same gradual manner. I cannot help 

 observing, that here the wise design of Providence is very con- 

 spicuous. Their appearance keeps pace with that of the in- 



* Ray on the Wisdom of God in the Creation, Part I. p. 128. 



sects 



