354 THE PRINCIPLES OF SCIENCE. 



in some cases and not in others. Now the name correla- 

 tion requires to be used with the same qualification. 

 Things are correlated (con, relata) when they are so re- 

 lated or bound to each other that where one is the other 

 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 correlated with inertia ; in 

 botany exogenous growth is correlated with the posses- 

 sion of two cotyledons, or the production of flowers with 

 that of spiral vessels. But it is in the classificatory 

 sciences especially that the word correlation has been em- 

 ployed. 



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 

 of bone, articulated with the squamosal element of the 

 skull, and also with the possession of mammae and non- 

 nucleated red blood-corpuscles. Professor Huxley remarks d 

 that this statement of the character of the class mammalia 

 is something more than an arbitrary definition ; it is a 

 statement of a law of correlation or co-existence of animal 

 structures, from which most important conclusions are 

 deducible. It involves a generalization to the effect that 

 in nature the structures mentioned are always found 

 associated together. This simply amounts to saying that 

 the formation of the class mammalia involves an act of 

 inductive discovery, and results in the establishment of 

 certain empirical laws of nature. Professor Huxley has 

 excellently expressed the mode in which discoveries of this 

 kind enable naturalists to make deductions or predictions 



d ' Lectures on the Elements of Comparative Anatomy, and on the 

 Classification of Animals/ 1864, p. 3. 



