70 LAY SERMONS, ESSAYS, AND REVIEWS. [v. 



which there is least to be learnt by mere observation, and that 

 which affords the greatest field for the exercise of those faculties 

 which characterise the experimental philosopher. I confess, if 

 any one were to ask me for a model application of the logic of 

 experiment, I should know no better work to put into his hands 

 than Bernard s late Researches on the Functions of the Liver. 1 



Not to give this lecture a too controversial tone, however, I 

 must only advert to one more doctrine, held by a thinker of our 

 own age and country, whose opinions are worthy of all respect. 

 It is, that the Biological sciences differ from all others, inas 

 much as in them classification takes place by type and not by 

 definition. 2 



It is said, in short, that a natural-history class is not capable 

 of being defined that the class Rosacese, for instance, or the 

 class of Fishes, is not accurately and absolutely definable, inas 

 much as its members will present exceptions to every possible 

 definition; and that the members of the class are united 

 together only by the circumstance that they are all more like 

 some imaginary average rose or average fish, than they resemble 

 anything else. 



But here, as before, I think the distinction has arisen entirely 

 from confusing a transitory imperfection with an essential 

 character. So long as our information concerning them is 

 imperfect, we class all objects together according to resemblances 

 which we feel, but cannot define ; we group them round types, in 

 short. Thus, if you ask an ordinary person what kinds of 



1 &quot; Nouvelle Fonction du Foie conside&amp;gt;6 comme organe producteur de 

 matiere sucree chez 1 Homme et les Animaux,&quot; par M. Claude Bernard. 



2 &quot; Natural Groups given by Type, not by Definition The class is 



steadily fixed, though not precisely limited ; it is given, though not circum 

 scribed ; it is determined, not by a boundary-line without, but by a central 

 point within ; not by what it strictly excludes, but what it eminently in 

 cludes ; by an example, not by a precept ; in short, instead of Definition we 

 have a Type for our director. A type is an example of any class, for instance, 

 a species of a genus, which is considered as eminently possessing the 

 characters of the class. All the species which have a greater affinity with 

 this type-species than with any others, form the genus, and are ranged about 

 it, deviating from it in various directions and different degrees.&quot; WHEWELL, 

 The Philosophy of the Inductive Sciences, vol. i. pp. 476, 477. 



