LIFE ON THE EARTH. 143 



periods in the northern zones of the earth, varied 

 by at least one great interval of remarkable cold in 

 later times. Before proceeding to consider how such 

 variations of climate might be possible, it will be 

 useful to collect some points of the evidence which 

 may be regarded as establishing the fact of their 

 having really taken place. 



There is only one kind of evidence that to be 

 obtained from organic remains ; and as in existing 

 nature some groups are more definitely related to 

 and indicative of climate than others, so in the fossil 

 world. In some modern genera and families the 

 species are distributed over different latitudes ; some 

 being intra-tropical, others extra-tropical, some in 

 the temperate, others in the arctic zones. 



Such genera and families can only be employed 

 in arguments on ancient climate, where they contain 

 species which are both recent and fossil, and this 

 occurs only in the Csenozoic Strata. But there are 

 other fossil genera, families, and even larger natural 

 groups, which on proper questioning yield satisfac- 

 tory answers in regard to the main characters of the 

 climate to which they were appointed, whether on 

 land or in the sea. 



To take our first examples from the vegetable 

 kingdom, we may inquire what climate is indicated 

 by the characteristic forms of land-plants buried in 



