234 THE AMERICAN NATURALIST [Vol. XLII 



believe them to have descended from a common stock, 

 constitute a species.'''' 



From the above definitions it will be at once evident 

 that there are two leading ideas in the minds of zool- 

 ogists when they employ the term species; one of these 

 being a certain amount of resemblance between indi- 

 viduals, and the other being the proof that the individuals 

 so resembling each other have descended from a single 

 pair, or from pairs exactly similar to one another. The 

 characters in which individuals must resemble one an- 

 other in order to entitle them to be grouped in a separate 

 species, according to Agassiz, ''are only those determin- 

 ing size, proportion, color, habits and relations to sur- 

 rounding circumstances and external objects." 



On a closer examination, however, it will be found that 

 these two leading ideas in the definition of species — ex- 

 ternal resemblance and community of descent— are both 

 defective, and liable to break down if rigidly applied. 

 Thus, there are in nature no assemblages of plants or 

 animals, usually grouped together into a single species, 

 the individuals of which exactly resemble one another 

 in every point. Every naturalist is compelled to admit 

 that the individuals which compose any so-called species, 

 whether of plants or of animals, differ from one another 

 to a greater or less extent, and in respects which may be 

 regarded as more or less important. The existence of 

 such individual differences is attested by the universal 

 employment of the terms ''varieties" and "races." 

 Thus a "variety" comprises all those individuals which 

 possess some distinctive peculiarity in common, but do 

 not differ in other respects from another set of individ- 

 uals sufficiently to entitle them to take rank as a separate 

 species. A "race," again, is simply a permanent or 

 "perpetuated" variety. The question, however, is this 

 —How far may these differences amongst individuals 

 obtain without necessitating their being placed in a 

 separate species? In other words: How great is the 

 amount of individual difference which is to be considered 



