On the Fertility of different Breeds of Sheep, tyc. 



99 



Cystopteris, Lindsaya, and the Pteridese. But this sequence is already 

 laid out in this order in the Synopsis, and it illustrates one at least of 

 the lines along which mixed forms are believed to have been derived 

 from the Gradatse. No attempt has been made to follow the natural 

 grouping of the Mixtas into detail, or to test the arrangement of them 

 in the Synopsis. Sufficient has, however, been said to show that the 

 systematic divisions of the ferns now proposed fall in readily with the 

 system of Sir William Hooker, notwithstanding that they are based 

 upon details of which he cannot have been aware. 



" Note on the Fertility of different Breeds of Sheep, with Bemarks 

 on the Prevalence of Abortion and Barrenness therein." By 

 Walter Heape, M.A., Trinity College, Cambridge. Communi- 

 cated by W. F. B. Weldon, F.B.S. Beceived March 9,— 

 Bead April 20, 1899. 



The importance of fertility as a factor in the survival of a species is 

 admirably demonstrated by Hafkine,* whilst Professor Karl Pearsonf 

 shows that fertility when correlated with other characteristics works a 

 progressive change, and that not only is fertility a race characteristic, 

 but may be a class characteristic in the human species. 



Among domesticated animals, although fertility ma}' be a racial 

 characteristic, its importance may be much reduced from a variety of 

 circumstances. 



Among sheep there is undoubted evidence of the racial character of 

 fertility, but the quality of the wool or the value of fat sheep of a par- 

 ticular breed may render that breed worth keeping in spite of a low 

 rate of fertility as compared with other breeds. Then the rate of fer- 

 tility may be artificially increased, as when certain rams of undoubted 

 value as progenitors, but useless as breeders if left to themselves, 

 become valuable sires by the help of the shepherd. In the same way a 

 certain breed of sheep, kept in one district and managed in a particular 

 manner, may be more liable to abortion, or to barrenness, or to mor- 

 tality among the lambs, than the same breed in another district 

 managed in another way, and yet the former may, on account of the 

 supply of food which it is possible to grow there per acre, prove the 

 most remunerative. 



From these and many other similar reasons the survival of a particular 

 breed or its retention in, or importation to, a particular district is not 

 necessarily due to natural fitness or adaptability. At the same time a 



* " Recherches sur l'adaptation au milieu chez les Inf usoires et les Bacteries ; 

 contribution a 1' etude de l'immunite," ' Annales de l'lnstitut Pasteur,' vol. 4, 1890. 



f " Contributions to the Mathematical Theory of Evolution. Note on Repro- 

 ductive Selection," ' Roy. Soc. Proc.,' vol. 59, 1896. 



