Classification and Distribution of Animals. 15 



reptiles, birds and quadrupeds, which have an internal 

 skeleton, a brain and vertebral column. This one 

 sub-kingdom includes the most complex of animals 

 whose structure requires more minute examination 

 than does that of the other sub-kingdoms. We will 

 in the present volume consider the seven invertebrate 

 sub-kingdoms. 



In comparing these sub-kingdoms, we speak of 

 forms as being high or low in organisation according 

 to the degree in which special parts are appropriated 

 for the discharge of special functions. We also notice 

 that no organ appears for the first time in animals in 

 a state of complexity, but on the contrary, there is 

 always in lower forms a prophetic foreshadowing of it 

 in the modification of some part already existing. 



Distribution. Every species of animal is limited to 

 a definite geographical area. Thus the earth's surface 

 may be divided into regions, each characterised by 

 special inhabitants, and the collected animals of any 

 region we speak of as its fauna. As a rule, life increases 

 in amount in any country with increasing, and dimi- 

 nishes with diminishing temperature. Tims the fauna 

 of a tropical exceeds that of a temperate region. The 

 number of animals is also larger when the difference 

 between the winter and summer temperature is small, 

 than in a country with the same mean temperature but 

 with a greater range between maximum and minimum. 

 Moisture is also favourable to animal life, and the 

 fauna of a moist exceeds that of a dry region, other 

 things being equal. 



Many animals live in places from which light is 

 excluded, as in caves ; these have rudimental eyes, and 



