646 ON SOME MARSUPIAL EMBRYOS. 



only 7 per cent, of the genera found on the plains of the Puna 

 are endemic. These facts are believed to indicate that the extent 

 of environmental change, expressed chiefly in climatic conditions, 

 is a more important evolutionary factor than those exerted by 

 time of isolation or extent of distance from the presumed ancestral 

 forms. 



Prof. J. P. Hill, F.R.S., Y.P.Z.S., exhibited, and made remarks 

 upon, a series of lantern-slides of some Marsupial embryos, 

 dealing especially with the Koala {Phascolarctos) and the Wombat 

 (^Fhascolomys). 



