4 INVERTEBRATE ZOOLOGY 



species can be only that scientific name under which it was first 

 described, provided that the name is binomial and has not been 

 used previously for some other animal. The same specific name 

 may be used for any number of different animals belonging to 

 different genera. 



The tenth edition of Linnaeus' Systema Naturae was published 

 in 1758. Since the appearance of this work marks the first 

 general application of binary nomenclature in zoology, this date 

 has been accepted by the International Commission as the 

 starting point for application of the law of priority. 



It frequently happens that after certain generic and specific 

 names come into general use some one discovers that an earlier 

 writer had applied a different binomial name to the same animal. 

 Then the generally accepted name has to be dropped in favor of 

 the prior name. Confusion and inconvenience frequently result 

 from the application of this law but it seems to be the only safe 

 means whereby scientific names may be accepted. 



It also frequently happens that a generic name has been used 

 previously for some other genus of animals and in this instance 

 only the earliest use of the name stands as valid, for the second 

 time the name is used it is considered as a homonym. The name 

 Trichina was applied to a parasitic worm in 1835, but in 1830 

 the same name had been assigned to a genus of insects. It 

 therefore became necessary to rename the worm, and even after 

 the name Trichina came into very general and popular use the 

 worm was renamed Trichinella. 



Since the name of a family is formed by adding the suffix -idae 

 to the stem of the name of the type genus, family names are also 

 governed by the same laws which govern the use of generic 

 names. 



Phylogenetic Relationships. — The arrangement of animal 

 groups in a list or in a book of necessity follows a sequence which 

 places the lowest at one end and the highest at the other. Fre- 

 quently, this arrangement carries with it the idea that classifica- 

 tion intends to express a linear relationship of all forms, that 

 each group has arisen from or has evolved from the one preceding. 

 Such a hypothesis was held by some of the early zoologists, but 

 the more commonly accepted idea of today postulates that our 

 present-day animals are not directly related but that two groups 

 bear closer or more distant relationship to each other chiefly 

 through an extinct form which is an ancestor of both. Thus, 



