328 OUTLINES OF EVOLUTIONARY BIOLOGY 



fact that the glaciation was not continuous but alternated with a 

 succession of warm periods. The southern hemisphere also 

 experienced a glacial epoch during which warm and cold periods 

 alternated, and astronomers hold that the warm periods in one 

 hemisphere coincided with cold ones in the other. It has been 

 calculated that each warm or cold period lasted for about 

 21,000 years. 



Other important changes in climate occurred long before the 

 great glacial epoch. Thus the fossil remains of a luxuriant 

 vegetation in Greenland and other northern localities indicate 

 the occurrence of a mild arctic climate in Miocene times. Such 

 a climate must have favoured migration between the old and 

 new worlds by way of what is now Behring Strait, which may 

 very well have been dry land at the time. 



Owing to the gradual loss of the earth's heat by radiation and 

 the consequent shrinkage and crumpling of the solid crust, 

 variations in the level of the land are constantly taking place. 

 Areas which are at present separated by sea may have been 

 connected in former times and vice versa, and there can be no 

 doubt that the distribution of plants and animals has been 

 profoundly influenced in this way. Many cases of discontinuity 

 in distribution may be explained by the former existence of land 

 connections which no longer remain. It is necessary, however, 

 to be extremely careful how we invoke the aid of this principle, 

 which, as an easy way out of difficulties, is apt to lead us into 

 all sorts of unjustifiable speculations. 



Those remarkable animals the lemurs, as a group, exhibit a 

 very curious discontinuity in their distribution, occurring in 

 Africa (and especially Madagascar) on the one hand and in 

 Southern Asia on the other. To explain this distribution it has 

 been suggested that in former times a continent Lemuria 

 existed in the Indian Ocean. Similarly, but with perhaps 

 greater justification, it is believed by many people that the 

 antarctic continent at one time extended much further north 

 than at the present day, so as to afford, possibly with the aid of 

 a chain of islands and with the co-operation of a mild antarctic 

 climate, a route along which migration might take place between 

 South America and Australasia. In this way may be explained 

 certain remarkable points of agreement between the fauna and 

 flora of Australia and New Zealand and those of South America. 

 The genus Fuchsia, for example, is typically South American, 



