VARIETY AND UNITY IN LIFE 



13 



varied. Pasteur has defined fermentation as "life without air."^ 

 A host of chemical changes in organic matter, fermentation, 

 putrefaction, infection of disease all these are the work of 

 minute organisms none the less real because invisible and as 

 varied in form and structure as in the differing effects their 

 presence may produce. 



Each kind of animal or plant, that is, each set of forms ^ 

 which in the changes of the ages has diverged tangibly from its 

 neighbors, is called a species. There is no absolute definition 

 for the word species. The word kind represents it exactly in 

 common language, and is just as susceptible to exact definition. 



Fi<;. 2. Kangaroo rat from the California-Mojave desert (one-half natural size). 



The scientific idea of species does not differ materially from the 

 popular notion. A kind of tree or bird or squirrel is a species. 

 Those individuals which agree very closely in structure and 

 function belong to the same species. There is no absolute test, 

 other than the common judgment of men competent to decide. 

 Naturalists recognize certain formal rules as assisting in such a 

 decision. A series of fully intergrading forms, however varied 

 at the extremes, is usually regarded as forming a single species. 

 There are certain recognized effects of climate, of climatic iso- 

 lation, and of the isolation of domestication. These do not 

 usually make it necessary to regard as distinct species the 

 extreme forms of a series concerned. 



In the words of the entomologist Rambur, " A species is a 

 group of beings which in successive generations show the same 

 characters of organization, unchanged so long as the locality 

 and external conditions remain unchanged.'' 



