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THE AMERICAN NATURALIST [Vol. XLII 



of their coworkers throughout the world. As early as 

 1884 this postulate was incoroprated in the A. 0. U. Code 

 of Nomenclature (first published in 1886), as a reason for 

 the adoption of trinomials, in the following words: 



Recognition of the scientific fact, that a " species," so-called, is not 

 a fixed and special creation, as long supposed, but simply a group of 

 the same intrinsic character as that called a genus, though usually less 

 extensive, and always of a lower taxonomic rank, has done more than 

 any other single thing to advance the science of zoology; for the whole 

 theory of evolution turns, as it were, upon this point. 



Respecting the meaning and function of the trinomial 

 system, it is said in the same connection : 



Trinomials are not necessarily to be used for those slightly dis- 

 tinct and scarcely stable forms which zoologists are in the habit of calling 

 "varieties"; still less for sports, hybrids, artificial breeds, and the like; 

 nor indeed to signalize some grade or degree of difference which it may 

 be, desired to note by name, but which is not deemed worthy of a 

 specific designation. The system proceeds upon a sound scientific prin- 

 ciple, underlying one of the most important zoological problems of the 

 day, — no less a problem than that of the variation of animals under 

 physical conditions of environment, and thus of the origin of species 

 itself. The system is also intimately connected with the whole subject 

 of the geographical distribution of animals; it being found, as a 

 matter of experience, that the trinomial system is particularly pertinent 

 and applicable to those geographical " subspecies," "races" or "varie- 

 ties" which have become recognizable as such through their modification 

 according to latitude, longitude, elevation, temperature, humidity, and 

 other climatic conditions. It is obvious, therefore, that the kind or 

 quality, not the degree or quantity, of difference of one organism from 

 another determines its fitness to be named trinomially rather than 

 binomially. In a word, inter gradation is the touchstone of tri- 



A small amount of difference, if constant, was con- 

 sidered "specific," in a proper sense, while a large 

 amount of difference, if found to lessen and disappear 

 when specimens from contiguous faunal areas were com- 

 pared, was considered as not specific. 1 



While these distinctions and principles have been 

 found to work well in vertebrates, and have become to a 



*Cf. A. O. U. Code of Nomenclature and Check-list of North American 

 Birds, 1st ed., 1886, pp. 27, 31, and context. 



