AS RESTING ON CLIMATE. 139 



Eurythermal animals axe of even less value in forming an 

 opinion on the subject, since it is well known that they are 

 especially characterised by the extraordinary adaptability that 

 they sometimes display to very different and remote extremes 

 of temperature. If we are bent on reconstructing the mode 

 of life of fossil forms, and their climatic conditions of existence, 

 by comparison with allied living forms, land-animals must be 

 decidedly preferred to water-animals ; but even these, as it seems 

 to me, offer absolutely no certain evidence. At most this mode 

 of comparison can only apply when the fossil and living ani- 

 mals are so closely similar that we are forced to regard them 

 as identical. This is known to be the case with the animals of 

 what is known as the Glacial Period ; but, as soon as we reach 

 the deeper strata, and the identity of the species with those now 

 living ceases,- our right to construct a theory of the climate of 

 past epochs by a comparison of fossil and living species entirely 

 disappears. The very generally received opinion that such a 

 reconstruction is possible rests in part on the old, but abso- 

 lutely false, idea that certain absolute degrees of warmth, and 

 partioulai-ly the mean annual temperature, have a definite effect 

 on the life of animals ; and, secondly, on the indisputable fact 

 that the climatic difference of two countries always goes hand 

 in hand with a dissimilarity in their fauna. But it ought not 

 to have been forgotten that the daily and annual variations of 

 temperature ars not the only means which Nature has had at 

 her disposal for the selection of species and the geographical 

 limitation or distribution of particular forms in successive 

 geological periods ; it ought to have been duly considered that, 

 when a change of temperature is introduced in any loc-ility, the 

 infliience, whether favourable or unfavourable, that it may have 

 on the mode of life or even on the existence of the animals 

 may often be completely neutralised by the effects** of other 

 conditions of existence in no way depending on the temperature 

 and its variations. 



