^S2 GENERAL PRINCIPLES 



a few familiar forms. Man is naturally immune to fowl cholera, 

 though sometimes attacked by cattle fever, anthrax. The fowl 

 is immune to rabies, to which both man and the dog are subject. 

 The dog is immune to anthrax. By an attack of measles or 

 smallpox man acquires immunity against subsequent attacks. 

 By vaccinating man with the virus of cowpox, a mild disease 

 is produced, which renders the individual actively immune 

 against smallpox. Passive immunity against diphtheria is se- 

 cured by the injection of an antitoxin serum taken from an 

 actively immune horse. 



EVOLUTION 



781 . Species. — The word species is one of the most important 

 and most frequently employed of all biological terms, and yet 

 it is impossible of exact definition. A species is a kind of a 

 plant or animal, using the word kind in its most common sense. 

 Thus the sweet-gum (Liquidambar Styracifiua), the persimmon 

 (Diospyros Virginiana), and the tulip tree (Liriodendron tulipi- 

 fera), are clearly defined species as are also, among animals, the 

 turkey vulture, or buzzard (Cathartes aura) and the robin 

 (Turdus migratorius). Any given example may immediately 

 be recognized as sweet-gum or not-sweet-gum, as robin or not- 

 robin. In other cases, however, difficulties may arise. Thus, 

 among the oaks we have the willow oak, the blackjack, the white 

 oak and the Spanish oak, all of which are distinct species and 

 readily distinguishable. But it frequently occurs that the 

 flowers of one species are pollinated by those of another and the 

 resulting offspring is called a hybrid. It resembles both parent 

 species to some extent, but belongs to neither. A much more 

 important difficulty arises from individual variation, for the 

 individuals of a species are never exactly alike. Even the in- 

 dividuals sprung from the same parents may vary greatly 

 among themselves. Allowance must, therefore, be made for 



