INTRODUCTION 11 



their distribution is determined largely by temperature 

 and is better marked than in the case of marine animals. 

 As far as the latter are concerned it is only when we are 

 dealing with modern species that we can, as a rule, speak 

 with any degree of certainty on this subject; this is 

 owing to the fact that at the present day the individual 

 species of the same genus have often a very different 

 distribution, some being found in warm, others in cold, 

 regions. Even when all the fossils in a formation belong 

 to extinct species, the assemblage of genera is sometimes 

 such as marks some region at the present day ; thus, for 

 example, in the London Clay we find that many of the 

 genera of mollusks are now characteristic of tropical or 

 subtropical seas. 



The study of fossil animals and plants is of the 

 highest importance to the biologist, not only because 

 they are the ancestors of modern species, but because 

 among fossil forms we find many groups {e.g. Graptolites, 

 Cystids, Blastoids, Trilobites, Eurypterids), which are 

 altogether extinct, and which often throw light on the 

 relationship of existing animals and plants. Others (e.g. 

 Crinoids, Brachiopods, Nautiloids) are represented at the 

 present day by few forms only, but were, in past ages, very 

 abundant ; consequently no adequate knowledge of such 

 groups of animals can be obtained from the study of living 

 examples only. In some cases the ancient forms serve to 

 connect groups which, at the present day, appear to be 

 quite distinct ; thus, for example, the earliest known bird 

 (Archoeopteryx, from the Solenhofen Limestone, Upper 

 Jurassic) shows, in several important characters, affinities 

 to the Reptiles. From the point of view of the biologist, 

 the greatest interest in Palaeontology is found in the bear- 

 ing it has on the subject of evolution : it is only by a study 



