Page:Popular Science Monthly Volume 9.djvu/434

410 every adjective denotes a species of the genus indicated by the substantive 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 individuals which corresponds with an early stage of that process of abstraction 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 characters 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 physical 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 unlikenesses of form and structure, until he has arranged them into groups of individuals which agree in certain constant characters and differ only by insignificant features, or by such peculiarities as vary in different individuals in such a manner that an insensible