INTRODUCTION n 



find birds with teeth, great aquatic reptiles as large as whales, 

 fishes, molluscs, Crustacea, etc., all of an entirely different 

 type from any now existing. We thus find that the former 

 were in many cases utterly unlike the present animal inhabi- 

 tants of the globe, and we arrive at the notion of a succession 

 of life in time, and are even able, in exceptionally favourable 

 circumstances, to trace back existing forms to their extinct 

 ancestors. 



By combining the results of comparative morphology, 

 embryology, and palaeontology we get a department of 

 Zoology called Phylogeny, the object of which is to trace 

 the pedigrees of the various groups. There are, however, 

 very few cases in which this can be done with any approach 

 to exactness; most " phylogenies " are purely hypothetical, 

 and merely represent the views at which a particular zoolo- 

 gist has arrived after a more or less exhaustive study of the 

 group under discussion. 



Animals may also be studied from the point of view of 

 Distribution. One aspect of this study is inseparable from 

 Palaeontology, since it is obviously necessary to mention in 

 connection with a fossil the particular system or systems of 

 rocks in which it occurs : thus we distinguish geological 

 distribution or distribution in time. 



The distribution of recent forms may be studied under 

 two aspects, their horizontal or geographical distribution, 

 and their vertical or bathymetrical distribution. To men- 

 tion the latter first, we find that some species exist only on 

 plains, others — hence called alpine forms — on the higher 

 mountains ; that some marine shells, fishes, etc., always keep 

 near the shore {littoral species), others live at great depths 

 {abyssal species), while others {pelagic species) swim on the 

 surface of the ocean. Among aquatic animals, moreover, 

 whether marine or fresh-water, three principal modes of life 



