2 INVERTEBRATE ZOOLOGY 



THE SYSTEM OF CLASSIFICATION 

 Primary and Subdivisions. — It should be recalled that each 

 phylum is made up of classes, orders, families, genera, and 

 species in descending sequence and that each primary division 

 is capable of still further arrangement into sub- and supergroups. 

 Thus, when a class comprises a large number of orders, it fre- 

 quently becomes convenient to combine the orders within the 

 single class into two or more groups by uniting those orders 

 which seem to have essential features in common into a subclass 

 or a superorder. Each major or primary group may thus have 

 within it secondary groups of greater scope than the next-lower 

 primary division. 



Basis for Classification. — The entire system of classification" 

 of animals is based upon interpretation and judgment. There 

 are no absolute units of measure which determine how many 

 species or how many classes any phylum is to include. Every 

 attempt at classification in some measure aims to express varying 

 degrees of relationship between different organisms, and every 

 decision regarding relationships is based upon fixed premises. 

 Proofs of phylogenetic relationships are not procurable through 

 direct observation, but evidences are offered in the structure and 

 development of the individual and frequently through fossil 

 remains of extinct forms which serve as bridges or connecting 

 links between our modern groups. Man's knowledge of all 

 organisms is, at best, but fragmentary. A comparison even of 

 the known facts of structure, development, and habits of two 

 organisms involves much interpretation in determining the 

 relative importance which is to be ascribed to each set of facts. 

 Some structures and organs are highly variable even among the 

 offspring of the same parents, while other characters may be 

 relatively fixed in members of one group and highly variable in 

 those of another. Obviously, then, all facts are not of equal 

 significance in determining relationship and there is no arbitrary 

 means of predicting which are of value and which are worthless 

 in making comparisons. 



In common parlance, one organism is said to be higher or 

 lower than another without any conscious analysis of how the 

 decision has been reached. Such an expression, in order to carry 

 weight, is reached only after numerous comparisons have been 

 made and a decision has been reached as to what differences are 

 essential and what incidental. 



