56 PRINCIPLES OF PALAEONTOLOGY. 



majority of instances are the remains of marine animals, it is 

 mostly the temperature of the sea which can alone be determined 

 in this way ; and it is important to remember that, owing to the 

 existence of heated currents, the marine climate of a given area 

 does not necessarily imply a correspondingly warm climate in 

 the neighboring land. Land-climates can only be determined by 

 the remains of land-animals or land-plants, and these are com- 

 paratively rare as fossils. It is also important to remember that all 

 conclusions on this head are really based upon the present dis- 

 tribution of animal and vegetable life on the globe, and are 

 therefore liable to be vitiated by the following considerations : 



a. Most fossils are extinct, and it is not certain that the 

 habits and requirements of any extinct animal were exactly 

 similar to those of its nearest living relative. 



b. When we get very far back in time, we meet with groups 

 of organisms so unlike anything we know at the present day as 

 to render all conjectures as to climate found upon their sup- 

 posed habits more or less uncertain and unsafe. 



c. In the case of marine animals, we are as yet very far 

 from knowing the exact limits of distribution of many species 

 within our present seas; so that conclusions drawn from living 

 forms as to extinct species are apt to prove incorrect. For 

 instance, it has recently been shown that many shells formerly 

 believed to be confined 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 conditions under which certain beds were 

 supposed to have been deposited. 



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 establishment 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 of con- 

 ducing to its extension. 



Still, we are in many cases able to draw completely reliable 

 conclusions as to the climate of a given geological period, by 

 an examination of the fossils belonging to that period. Among 



