174 OUTLINES OF CLASSIFICATION. 



The fern, Pteris aquilina, is similarly a member of the 

 genus known as Pteris, which includes many species (aquilina, 

 hastata, cretica, etc.). Pteris is but one of many genera which 

 are associated to form a larger group, Vascular Cryptogams (often 

 called JPteridophyta), which is one member of the great group 

 Metaphyta, including all many-celled plants ; and this in turn is 

 a member of the group of plants in general. 



In this manner the great primary group of living things may 

 be successively subdivided into secondary, tertiary, quaternary 

 groups, etc. ; and the study of any single organism, as a bird, 

 therefore, teaches us not only a great deal of the smaller group, 

 A.ves, to which it immediately belongs, but also much of the 

 larger group Vertebrate^ which includes the Aves, and something 

 of the still broader group, Metazoa, and of Animal i a in general. 

 Thus regarded, every living thing is full of meaning as a repre- 

 sentative of other living things more or less nearly like itself. 



Basis of Classification. Living things may be classified by 

 their resemblances in structure, in function, or in both. But ex- 

 perience has shown that no consistent classification can be founded 

 on structure alone, or on function alone, and that both factors 

 must be considered in any natural system. It has been pointed 

 out at the close of the last chapter that the only fundamental dif- 

 ference between the fern and the earthworm lies in the mode of 

 nutrition in other words, it is a difference of function. This 

 particular plant and animal are to a great extent typical of plants 

 and animals generally. Broadly speaking, all plants agree with 

 the fern in being able to construct proteid matter out of much 

 simpler compounds, while all animals lack this power, and must 

 be supplied with proteid matter as part of their food. In the 

 last analysis this is the only point which will distinguish all 

 plants from all animals. And hence the arrangement of all 

 living things into the two primary groups of plants and animals 

 is based upon differences of function, and not of structure. 



The case is different, however, with the subdivisions of plants 

 and animals ; for, with few exceptions, it is found necessary to 

 found these upon peculiarities of structure (using this word to 

 include development) rather than of function. For example, 

 birds and butterflies agree functionally in possessing the power of 

 flight by means of wings. But the wings of the two animals are 

 totally unlike in structure, although performing the same function. 



