14 GENERAL SURVEY OF THE ANIMAL KINGDOM. 
(a) Rhizopods, with outflowing threads or processes of living mattey, 
é.g. the chalk-forming Foraminifera (Fig. 17). 
(6) 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 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 
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,” z.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, families, orders, 
‘classes, and so on. 
To give an illustration, all the tigers are said to form the species 
Felis tégrés, of the genus Fes, in the family Felide, 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 eé/is ; broader still are the resemblances between all members 
of the cat family Felidz ; still wider those between cats, dogs, bears, 
and seals, which form the order Carnivora; and Jastly, 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 :— 
