HISTORICAL PREFACE xxxi. 



be that this one Gannet reaches a very advanced age — one, 

 two, or three hundred years ! This has been justly spoken 

 of by authors as the potential longevity of- a species, 

 which is a very different thing from the average length 

 of a bird's life. A Gannet which has escaped all dangers 

 for a hundred years is an exemphfication of potential 

 longevity ; it has, as Dr. Chalmers Mitchell puts it, been 

 fortunate enough to go through life in an environment 

 relatively ideal. Such birds can have little or no effect 

 on the good of their species, the welfare of which is governed 

 by, and dependant upon, the average age to which its 

 members attain. 



Let us consider what knowledge we possess in this matter, 

 albeit it is assumption with which we have to be content, 

 rather than actual knowledge. To begin with, we are 

 surely justified in assuming that where there is a heavy 

 mortality among the young of a species, as is evidently the 

 case with the Gannet, we may look for long duration of life. 



This would have to be so in order to ensure replace- 

 ment of the parent Gannets by successfully-reared young 

 ones. 



August Weismann held that the duration of a bird's 

 life was determined by the length or shortness of the period 

 needed to rear enough offspring to perpetuate the race 

 (" Ueber das Dauer des Lebens," 1881 ; translation 

 " Essays upon Heredity," 1889), yet on the other hand 

 there must not be too many young reared, to maintain 

 about the same numerical level year by year. 



There are at least six other reasons for holding the 

 Gannet to be a bird which can live for manv vears. 



