40 THE SMALLEST LIVING THINGS 



damental protoplasm be changed drastically thereby, so that the 

 derived organization also becomes modified as indicated by struc- 

 tural peculiarities, a new form may result. New types arising 

 in this manner, if they breed true, are called mutations * and 

 have the value of new species. It is this principle which is the 

 basis of modern genetics, or the study of heredity and related 

 subjects. 



Adaptation to the environment is thus an outcome of the 

 irritability of protoplasm. New species may originate, either 

 ( 1 ) by permanent changes in the environment, or (2) by per- 

 manent changes in the fundamental organization. In either case 

 the conditions necessary for evolution are provided. 



Biological Classification*" 



Animals and plants are classified on the basis of their derived 

 organizations. Organisms having common basic structural char- 

 acters are placed together in one great group, known as a phylum 

 (for example, the phylum Vertebrata consisting of the backboned 

 animals) ; this phylum is subdivided into secondary groups — 

 classes — each with common characters subordinate to the pos- 

 session of a vertebral column {e.g., fishes, amphibia, reptiles, 

 birds, mammals). Each of these classes is further divided into 

 orders, and the orders into families; frequently there are sub- 

 orders and sub-families. All this dividing and sub-dividing is 

 on the basis of common structural characters. Finally, we come 

 to genera and then species — minor groups having certain 

 permanent modifications of the structures common to the 

 larger groups. In every case the derived organization must be 

 represented by a certain specific type of fundamental organiza- 

 tion of the protoplasm, which, upon development, gives rise to 

 the adult with the special structures characteristic of its derived 

 organization. In all vertebrates, for example, there must be 

 some combination in the fundamental organization that is ca- 

 pable, upon development, of giving rise to the vertebral column, 

 and all vertebrates must have this in common. But in addition 

 to this, amphibia and reptiles must have some combination in 

 their fundamental organization that is lacking in fishes, and 



* See "Heredity and Variation," pages 85-91, in this Series. 



t See "The Coming and Evolution of Life," Chapter V, in this Series. 



