14*2 THE CANADIAN ENTOMOLOGIST 



may be positively formulated, often numerically stated. For the present, 

 we will admit that these differences, thus accurately expressed, do really 

 exist, and depend upon measurements which may be repeatedly verified. 



With the first of these items we need do little more than present a few 

 illustrations ; the array in parallel columns will usually bring them into 

 sufficient prominence. The second is of more consequence, for it raises 

 the important question upon which this whole discussion will turn— Are 

 these differences of such a character as will warrant the erection of distinct 

 genera ? Mr. Scudder has already answered this question in the affirm- 

 ative ; for us to answer blankly in the negative would be to pit our opinion 

 against his, in which case the weight of authority would very largely and 

 very properly lie on his side. 



We must, therefore, briefly inquire into the distinctions which exist 

 between genera and species, as found in law and in usage. 



Probably we can appeal to no higher authority upon the law than that 

 of Agassiz, and accordingly we quote his definitions as found in the 

 " Essay on Classification." 



" Genera are most closely allied groups of animals differing neither in 

 form nor in complication of structure, but simply in the ultimate struc- 

 tural peculiarities of some of the parts." Eng. Ed., p. 249. 



" Genera [are] characterized by ultimate peculiarities of structure in 

 the parts of the body. 



" Species [are] characterized by relations and proportions of parts 

 among themselves, and of the individuals to one another and to the 

 surrounding mediums.'' P. 265. 



Here the question turns upon the force of the words " ultimate struc- 

 tural peculiarities." Can they mean that any difference which can be 

 formulated in the ratio of length to breadth in the same part, or of length 

 of one part to length of another part, is a difference of ultimate struc- 

 ture ? If one insect has its fore-tibia five-sixths the length of its fore- 

 femur, while another has the 'same parts in the ratio of four-sixths, or 

 six-sixths, are they for this cause of different genera ? Does this principle 

 extend through zoology ? Is Gen. Sheridan, who is short and stout, and 

 who, according to Pres. Lincoln, can scratch his ancle without stooping, 

 generically different from Gen. Sherman, who is tall and slender, and 

 whose ancles are evidently out of his reach ? Can any one safely affirm 

 of any individual of any species of any genus in the whole realm of 

 nature, that all its ratios of measurement in all its members are identical 



