io GENERAL SURVEY OF THE ANIMAL KINGDOM. 



have no organs, nor sexual reproduction in the ordinary 



sense of the phrase. The series includes 



. (a) Infusorians, with actively moving lashes of living 



matter ; 

 (fr) Rhizopods, with outflowing threads or processes of 



living matter ; 



(c) Gregarines, parasitic forms, without either lashes or 

 outflowing processes. 



Note on Classification. 



We naturally group together in the mind those impressions which are 

 like one another. In this lies 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 

 natural relationships of animals, to group together those which resemble 

 one another in their real nature or structure. It must therefore be 

 based on the results of comparative anatomy, technically speaking, on 

 " homologies," or real resemblances of structure. Whales must not be 

 ranked with fishes, nor bats with birds. 



To a classification based on structural resemblances, two corrobora- 

 tions are necessary, from embryology and from palaeontology. 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, families, 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 Felidae, in the order 

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

 are exceedingly close ; well-marked, but not so close, are the resem- 

 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 Felidse ; 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. 



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

 and that a species is a subjective conception within which we include 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 



