8 ZOOLOGY 



and paleontology we get a department of Zoology called Phylo- 

 geny, 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 zoologist 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 con- 

 nection 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 mention 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, &c., 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 are to be distinguished. These are animals, such 

 as Jelly-fishes, which float on or near in the water, and are 

 carried about passively by currents: such forms are included 

 under the term Plankton. Most Fishes, Whales, and Cuttle-fishes, 

 on the other hand, are strong swimmers, and are able to traverse 

 the water at will in any direction ; they together constitute the 

 Xi-l'ton. Finally, such animals as Crabs, Oysters, Sponges, Zoo- 

 phytes, &c., remain permanently fixed to or creep over the surface 

 of the bottom, and are grouped together, as the Benthos. 



Under the head of geographical distribution we have such facts 

 as the absence of all Land-mammals, except Bats, in New Zealand 

 and the Polynesian Islands, the presence of pouched Mammals, 

 such as Kangaroos and Opossums, only in some parts of America 

 and in Australia and the adjacent islands, the entire absence of 

 Finches in Australasia, and so on. We find, in fact, that the 

 fauna i.e. the total animal inhabitants of a country is to a 

 large extent independent of climate, and that the fauna? of 

 adjacent countries often differ widely. In fact, it is convenient 

 in studying the geographical distribution of animals largely to 

 ignore the ordinary division into continents, and to divide the 

 land-surface of the globe into what are called zoo-geographical 

 regions. The characteristics of these regions will be discussed in 

 a future section ; at present it is only necessary, for convenience of 

 reference, to give their names and boundaries. 



