Classification and Distribution of Animals. 13 



or ground parrot of New Zealand, which can hardly fly, 

 is in plumage like the mottled green vegetation among 

 which it lives. The ptarmigan and other birds become 

 white in winter, so as to become inconspicuous among 

 the snow. Sometimes an insect mimics in appearance 

 another of different nature living in the same district. 

 In such cases the insect imitated is one which, from its 

 disagreeable secretions or sting, is not a favourite prey 

 of insect-eaters. Hence the mimicry protects the 

 imitator, who is usually rarer than the insect imitated. 



Organs which are homologous consist of homo- 

 logous parts ; and as this is not the case in organs re- 

 sembling each other only in function, we must be 

 careful to discriminate morphological from physio- 

 logical likeness. 



In animals which consist of successive segments in 

 a chain, like centipedes or lobsters, each segment is 

 composed of parts similar to those of its neighbouring 

 joints. Such parts are said to be serial homologues, 

 as for example the fore and hinder limbs of quad- 

 rupeds. 



CHAPTER III. 



CLASSIFICATION AND DISTRIBUTION OF ANIMALS. 



Sab-Kingdoms. THE animal kingdom includes eight 

 sub-kingdoms. In these we observe a certain pro- 

 gressive increase in complexity, from one end of the 

 series to the other ; but they do not make a linear 

 series, as the highest organism of each is in no degree 

 related to the lowest organism of the next sub-kingdom, 

 being usually much more advanced and specialised, so 



