GROUPING OF LIVING THINGS. 175 



For this reason the bird and the butterfly are not placed in the 

 same group, since it can be shown that the likeness in function 

 is less important than the difference in structure. The same 

 principle holds true of nearly all kinds of plants and animals, and 

 hence biologists are agreed that a natural classification must rest in 

 the main upon structural characters; that is, it must be a morpho- 

 logical classification. There are, however, certain doubtful cases 

 (e.g., Bacteria) in respect to which biologists are not yet agreed 

 whether structure or function forms the most available basis of 

 classification. 



Natural and Artificial Systems. Morphological likeness or dif- 

 ference is therefore the usual basis of classification, but it is im- 

 portant to determine what particular likenesses or differences shall 

 be taken as characteristic of the various groups. It is easy to 

 show by a single example that morphological characters have 

 very unequal values when regarded from this stand-point. It 

 would be possible to classify all living things according to color, 

 as white, yellow, green organisms, etc. Such a classification 

 would, however, be artificial and destitute of scientific value be- 

 cause based upon a purely superficial and highly inconstant char- 

 acter. An interesting example of an artificial classification for- 

 merly employed is the system of Linnaeus, who classified flower- 

 ing plants into Mbnandria, Diandria, Triandria, Tetrandria, 

 etc., according to the number of stamens. This was sufficiently 

 convenient for a first, rough arrangement, but was soon found to 

 lead to the most incongruous association of plants agreeing in 

 the number of stamens, but differing in almost all other charac- 

 ters. From such cases it is plain that plants and animals cannot 

 be naturally classified by likeness or difference in a single charac- 

 ter arbitrarily selected. The entire organism must be taken into 

 account, and the natural classification differs from an artificial one 

 in representing real relationship, and not merely a superficial like- 

 ness. Modern biology teaches that this relationship is of pre- 

 cisely the same kind as human relationship, i.e., that it is due to 

 community of descent from ancestral plants or animals. 



The origin of plants and animals by descent from other pre- 

 existing forms can no longer be questioned ; and it is evident 

 that the origin of any given plant or animal has been a definite 

 process, to be determined like any other fact of natural history. 

 It is plain, therefore, that there can be but one natural system of 



