80 INTRODUCTION. 



recently been shown that many shells formerly believed to be con- 

 fined to the arctic seas have, by reason of the extension of polar 

 currents, a wide range to the south ; and this has thrown doubt 

 upon the conclusions drawn from fossil shells as to the arctic con- 

 ditions under which certain beds were supposed to have been de- 

 posited. 



d. The distribution of animals at the present day is certainly 

 dependent upon other conditions beside climate alone ; and the 

 causes which now limit the range of given animals are certainly 

 such as belong to the existing order of things. But the establish- 

 ment of the present order of things does not date back in many 

 cases to the introduction of the present species of animals. Even 

 in the case, therefore, of existing species of animals, it can often be 

 shown that the past distribution of the species was different formerly 

 to what it is now, not necessarily because the climate has changed, 

 but because of the alteration of other conditions essential to the 

 life of the species or conducing to its extension. 



Upon the whole, therefore, it would seem that conclusions as to 

 the climate of any particular area at any given point of geological 

 time must be accepted with considerable caution, unless in cases 

 where there happens to be direct physical evidence of an arctic 

 climate. It has, in fact, been even questioned that there existed 

 marked differences in the climate of different regions of the earth in 

 the earlier periods of the earth's history. Rather, it has been held 

 that in ancient geological times an equable temperature reigned 

 over the whole globe, as the result of a relatively high internal 

 terrestrial heat, the earth's surface and the air being thus main- 

 tained at a temperature sufficiently high to render the influence of 

 the sun's rays of comparatively little importance. On this view it 

 was supposed that it was only when the earth's internal heat had 

 been largely dissipated by radiation, that climatic zones were devel- 

 oped ; the temperature of each region coming ultimately to depend 

 mainly upon the amount of heat which it might receive, directly or 

 indirectly, from the sun. The change from the one condition of 

 things to the other was usually supposed to have corresponded, in 

 a general way, with the commencement of the Tertiary period. 



Apart, however, from the inherent improbabilities attaching to this 

 theory — so far, at any rate, as concerns the periods subsequent to the 

 introduction of animal and vegetable life upon the globe — it has 

 been shown by Professor Neumayr that the existence of definite 

 climatic zones can be demonstrated, by the evidence of fossils, in 

 periods at least as ancient as the Jurassic. As regards the Palae- 

 ozoic period, the principal argument for the assumption that the 

 earth enjoyed a uniformly high temperature, as pointed out by 

 Neumayr, is that the Palaeozoic animals, even in northern latitudes, 



