STRUCTURAL RELATIONSHIP 309 



eight legs, etc. Each class, in turn, is composed of 

 Orders into which closely related Families fall, and each 

 family is composed of Genera, which in turn embrace the 

 smallest groups or Species. 



Each group is arbitrary, but the characters upon which 

 the larger groups are founded have been shown by ex- 

 perience to be so constant as to permit of little present 

 modification. The changing groups are the families, 

 genera, and species, and of these the genera and species 

 are subject to the greatest mobility. There is no fixed 

 opinion as to what shall constitute a species or what shall 

 be called a variety of a species. In groups of organisms 

 much studied specific differences are so minute that only 

 the most careful scrutiny with the microscope can dis- 

 cover them; in groups little studied the species differ 

 quite as widely as the genera of much studied groups. 



Cuvier tried to make the criterion of specific dif- 

 ferentiation the tendency of the organism to breed true, 

 but as the breeding of vast numbers of organisms is 

 something concerning which no information is available 

 and upon which none may be attainable, it becomes 

 impossible to follow the suggestion. 



According to the general acceptation, a species is 

 composed of individual organisms whose dissimilarities 

 are so slight and inconstant as not to be definite. 

 Many think that specific differences should be struc- 

 tural only; others admit differences of coloration as marks 

 of specific differentiation. Those who base specific 

 characters upon structural differences alone, separate 

 similarly constructed but differently colored or different- 

 sized organisms into still lower groups known as varieties. 

 Varieties, however, may differ among themselves in 

 structure as species sometimes do. Thus, among the 

 barnyard fowls the rose comb and the toothed comb 

 and the presence or absence of spurs are well-marked 

 structural differences, yet are not looked upon as specific, 

 and no one can gainsay that there are structural dif- 

 ferences between the bull-dog, the greyhound, and the 



