CLASSIFICATION. 681 



to the building of the ships. The age of ships or other 

 structures is usually correlated with their general form, 

 nature of materials, &c, so that ships of the same name will 

 often resemble each other in many points. 



It is impossible to examine the details of some of the 

 so-called artificial systems of classification of plants, 

 without finding that many of the classes are natural in 

 character. Thus in Tournefort's arrangement, depending 

 almost entirely on the formation of the corolla, we find 

 the natural orders of the Labiatae, Cruciferse, Rosaceae, 

 Umbel! iferse, Liliaceae, and Papilionacece, recognised in his 

 4th, 5th, 6th, /th, Qth, and loth classes. Many of the 

 classes in Linnaeus' celebrated sexual system also approxi- 

 mate to natural classes. 



Correlation of Properties. 



Habits and usages of language are apt to lead us into 

 the error of imagining that when we employ different 

 words we always mean different things. In introducing the 

 subject of classification nominally I was careful to draw 

 the reader's attention to the fact that all reasoning and all 

 operations of scientific method really involve classification, 

 though we are accustomed to use the name in some cases 

 and not in others. The name correlation requires to be 

 used with the same qualification. Things are correlated 

 (con, relata) when they are so related or bound to each 

 other that ivhere one is the oilier is, and where one is not the 

 other is not. Throughout this work we have then been 

 dealing with correlations. In geometry the occurrence 

 of three equal angles in a triangle is correlated with the 

 existence of three equal sides ; in physics gravity is corre- 

 lated with inertia ; in botany exogenous growth is correlated 

 with the possession of two cotyledons, or the production 

 of flowers with that of spiral vessels. Wherever a propo- 

 sition of the form A = B is true there correlation exists. 

 But it is in the classificatory sciences especially that 

 the word correlation has been employed. 



We find it stated that in the class Mammalia the 

 possession of two occipital condyles, with a well-ossified 

 basi-occipital, is correlated with the possession of man- 

 dibles, each ramus of which is composed of a single piece 



