306 BIOLOGY: GENERAL AND MEDICAL 



at the present time not less than 500,000 different species 

 of living animals and about 75,000 different species of 

 fossil animals. Reflection upon the conditions of evolu- 

 tion considered in another chapter should convince one 

 that the number of species now living must be infini- 

 tesimal compared with the great number of extinct forms 

 from which they have descended and to which they are 

 related. In consequence the genealogical tree is deficient 

 in many of its parts which can be linked together only in 

 imagination. Some of these gaps can, and no doubt 

 will, be filled in time by the discovery of additional fossil 

 species, but many of them, embracing soft-bodied ani- 

 mals that leave no relics behind them, can never be 

 filled in. 



An important improvement in classification came 

 from the great French naturalist, Cuvier (1798). The 

 chief divisions are as follows: 



Branch I. Animalia Vertebrata: 

 Class 1. Mammalia 



2. Aves 



3. Reptilia 



4. Pisces 

 Branch II. Animalia Mollusca: 



Class 1. Cephalopoda 



2. Pteropoda 



3. Gasteropoda 



4. Acephala 



5. Brachiopoda 



6. Cirrhopoda 

 Branch III. Animalia Articulata: 



Class 1. Annelides 



2. Crustacea 



3. Arachnides 



4. Insects 

 Branch IV. Animalia Radiata: 



Class 1. Echinoderms 



2. Intestinal worms 



