114 BRITISH BIRDS. [vol. vii. 



The Sulidae appear to me to illustrate clearly how 

 this reduction has been brought about in their case. 

 We see that in some species — e.g. S. leucogaster, S. 

 cyanops — two eggs are the normal clutch. But it is 

 exceptional for more than one young to be reared. 

 Of seven hundred and forty nests only two contained 

 two young. W. K. Fisher found a nest with two newly 

 hatched young chicks, one of which had already been 

 trampled to death. It is then very unusual for the 

 chick which appears last to survive though it may 

 hatch soon after the first. The usual interval between 

 the deposition of the eggs was found to be a week : 

 for such a space of time to elapse before the completion 

 of a clutch of two eggs is abnormal, and it appears 

 highly probable that this is a preliminary to the birds 

 ceasing to deposit the second egg. A large proportion 

 of the second eggs laid was found to be infertile, and 

 this is undoubtedly a sign of decadence. 



Now in the case of S. leucogaster about ninety-eight 

 per cent, of the birds had young. Thirty-five nests 

 were found containing eggs alone, of these twenty-one 

 held two eggs, and fourteen held only one. Though 

 it was suggested that in some if not most of these latter 

 cases another egg might have been laid, it appears 

 certain, considering how late in the season these 

 observations were made, that a single egg sometimes 

 is all that is deposited by S. leucogaster. This shows 

 that the reduction from two eggs to one has actually 

 taken place in some individuals of this species. 



In other members of the family no trace remains of 

 the stage w^hen the normal clutch was two eggs. As an 

 instance S. piscator may be mentioned, and the Gannet, 

 8. hassana, invariably lays a single egg only. 



