14 GENERAL SURVEY OF THE ANIMAL KINGDOM 



(a) Rhizopods, with outflowing threads or processes of hving matter, 

 e.g. the chalk-forming Foraminifera (Fig. 17). 



(b) Infusorians, with actively moving lashes of living matter. 



(c) Sporozoa, parasitic forms, usually without either lashes or out- 

 flowing processes. 



Note on Classification 



We always group together in our mind those impressions which 

 are hke one another. In this hes the beginning of all classification, 

 whether that of the child, the savage, or the zoologist. For there are 

 many possible classifications, varying according to their purpose, 

 according to the points of similarity which have been selected as 

 important. Thus we may classify animals according to their habitats 

 or their diet, without taking any thought of their structure. 



But a strictly zoological classification is one which seeks to show the 

 blood-relationships of animals, to group together those whose affinities 

 are shown by their being like one another in architecture or structure. 

 It must, therefore, be based on the results of comparative anatomy — 

 technically speaking, on " homologies," i.e. resemblances in funda- 

 mental structure and in mode of development. Whales must not be 

 ranked with fishes, nor bats with birds. 



To a classification based on structural resemblances, two corrobora- 

 tions are of value, from embryology and from paleontology. On the 

 one hand, the development of the forms in question must be studied : 

 thus no one dreamed that a Tunicate was a Vertebrate until its life- 

 history was worked out. On the other hand, the past history must be 

 inquired into : thus the affinity between Birds and Reptiles is confirmed 

 by a knowledge of the extinct forms. 



In classification it is convenient to recognise certain grades or degrees 

 of resemblance, which are spoken of as species, genera, famihes, orders, 

 classes, and so on. 



To give an illustration, all the tigers are said to form the species 

 Felis tigris, of the genus Felis, in the family FeHdae, in the order 

 Carnivora, within the class Mammalia. The resemblances of all tigers 

 are exceedingly close ; well marked, but not so close, are the resera 

 blances between tigers, lions, jaguars, pumas, cats, etc., which form 

 the genus Felis ; broader still are the resemblances between all members 

 of the cat family Felidae ; still wider those between cats, dogs, bears, 

 and seals, which form the order Carnivora ; and lastly, there are the 

 general resemblances of structure which bind Mammals together in 

 contrast to Birds or Reptiles, though all are included in the series or 

 phylum Vertebrata. 



It must be understood that the real things are the individual animals, 

 and that a species includes all those individuals who resemble one 

 another so closely that we feel we need a specific name applicable to 

 them all. And as resemblances which seem important to one naturalist 

 may seem trivial to others, there are often wide differences of opinion as 

 to the number of species which a genus contains. 



But while no rigid definition can be given of a species, certain 

 common-sense considerations should be borne in mind : — 



