4 io THE POPULAR SCIENCE MONTHLY. 



every adjective denotes a species of the genus indicated by the sub- 

 stantive to which it is applied. 



In the technology of the physical sciences the term " species " has 

 a more restricted signification. It is used to denote a group of indi- 

 viduals which corresponds with an early stage of that process of ab- 

 straction by which the qualities of individual objects are arranged in 

 the subordinated categories of classification. 



The individual object alone exists in Nature ; but, when individual 

 objects are compared, it is found that many agree in all those charac- 

 ters which, for the particular purpose of the classifier, are regarded 

 as important, while they differ only in those which are unimportant ; 

 and those which thus agree constitute a species, the definition of 

 which is a statement of the common characters of the individuals 

 which compose the species. 



Again, when the species thus established are compared, certain of 

 them are found to agree with one another, and to differ from all the 

 rest in some one or more peculiarities. They thus form a group, 

 which, logically, is merely a species of higher order, while technically 

 it is termed a " genus." And, by a continuation of the same process, 

 genera are grouped into families, families into orders, and so on. 

 Each of the groups thus named is in the logical sense a genus, of which 

 the next lower groups constitute the species. 



The characters on which species are based necessarily depend upon 

 the nature of the bodies classified. Thus, mineral species are founded 

 upon purely morphological characters ; that is to say, they are defined 

 by peculiarities either of form, color, and the like, or of structure, which 

 last term may be used to include both the j)hysical and the chemical 

 characteristics of a mineral. The distinction between a species and a 

 variety is wholly arbitrary, except so far as it is commonly agreed that 

 individuals which differ from others only as terms of a gradual series 

 of modifications belong to the same species, and are to be considered 

 merely as varieties of that species. 



It is conceivable that animals and plants should have been known 

 to us only by their remains preserved in museums or in the fossil state. 

 If this had been the case, biological, like mineralogical species, could 

 have been defined only by morphological characters ; that is to say, 

 by the peculiarities of their outward form and inward structure ; and, 

 as a matter of fact, this is the state of our knowledge in respect of a 

 large proportion of the existing fauna and flora of the world, and of 

 all extinct animals and plants. 



A botanist or a conchologist who sets to work to arrange a newly- 

 received collection sorts his plants or his shells out according to their 

 likenesses and unlikenesscs of form and structure, until he has arranged 

 them into groups of individuals which agree in certain constant char- 

 acters and differ only by insignificant features, or by such peculiarities 

 as vary in different individuals in such a manner that an insensible 



