29 16 Proceedings of the Royal Irish Academy. 



included as part of his theory as to the origin of this group a land-connexion 

 between the Spanish peninsula and the British Islands. It seems to me that 

 even if this group survived the Glacial period in the west of Ireland there is 

 no reason why it should have reached, and no evidence that it did reach, this 

 country from that direction and in the alternative there is some reason for 

 saying that no land-connexion would have been of any use to it. 



Now climate has always been recognized as one of the chief factors in 

 limiting the area of distribution of a species, in fact we rely upon it to such 

 an extent that we deduce the character of the climate of a country in past 

 ages from the nature of the species which inhabited it. We find that, in 

 Eocene and Miocene times, the lion, hippopotamus and other animals which 

 now live in tropical countries inhabited the British Isles and we conclude 

 that our country at that time enjoyed a hot climate. "We know also that 

 during the same period the Arctic Begions were occupied by species now found 

 in temperate latitudes and we conclude that the climate of the Polar Begions 

 was then temperate. 1 Further, we can trace from the early Tertiary period 

 to the early Pleistocene or post-Pliocene a gradual change in the faunas and 

 floras of the northern hemisphere such that we conclude there was a gradual 

 cooling of the climate over the whole area until the British Isles had slowly 

 changed from hot to temperate and from temperate to cold, and until they 

 endured the rigours of what we now call an " arctic " climate. Geological 

 evidence as to the Glacial period supports the palaeontological record so that 

 the relationship of climate to fauna and flora, at any rate for that epoch, is 

 fully established. 



On the basis of this relationship between fauna and climate, every species 

 may be regarded as having a climatic area which it is capable of occupying. 

 About some centre within that area the conditions of existence will approach 

 the ideal for the species but towards the limits the conditions will become 

 less and less suitable. Further, this climatic area will consist of large habitable 

 areas dotted with uninhabitable " islands " and also, especially towards the 

 limits, of isolated patches or "islands" in the midst of uninhabitable tracts. 

 Fluctuations of climate will affect the area, isolating new patches and 

 bridging old gaps ; they will affect the rate of production of the species and of 

 competitive species so that there will be constant changes in the oecological 

 relations. The centre of production will however normally produce an excess 

 of individuals and the surplus will distribute itself, replacing losses and 

 spreading as far as climate and competition will allow. 



With changes in climate such as those which led up to the Glacial period, 



1 Nalh'irst, A. G. : " Fossil Floras of the Arctic Regions as evidence of Geological Climates." 

 Geol. Mag., N.S., Decade V, vol. viii, pp. 217-225. May, 1911. 



