1 3 o THE SCIENCE OF LOGIC. 



these were regarded as &quot; fixed &quot; in the sense that no one could originate, or be 

 itself an off-shoot of, another or other groups ; though each was recognized to 

 be capable of &quot; varying &quot; within certain limits, thus giving rise to &quot;varieties &quot; 

 within its own sphere. This &quot;fixity&quot; of any group of living organisms, the 

 fact that its members continue to &quot; breed true,&quot; to retain its characteristics 

 and to resist all attempts to merge these, by interbreeding, with those of ad 

 jacent groups, became the test of a &quot;natural kind&quot; or &quot;species,&quot; as distinct 

 from a &quot; variety,&quot; among organisms. For the fixed and unchangeable groups 

 botanists and zoologists adopted the name &quot; species &quot; (though &quot; genus &quot; would 

 have been more naturally suggested by the idea of descent from a common 

 stock), while to the smaller, changeable groups within each &quot;species,&quot; they 

 gave the name &quot; varieties &quot;. The higher and wider groups into which the 

 species themselves were gathered on account of their varying degrees of 

 similarity in structure, function, and general constitution, were called 

 &quot;genera&quot; &quot; natural orders,&quot; etc. (46). This relationship of similarity between 

 the various &quot;natural kinds&quot; was described as &quot;affinity&quot;: at first, perhaps, 

 without any suspicion of a real affinity, i.e. a connexion by descent from a 

 common stock, the term being used in a merely analogical sense, based on 

 the resemblances that are so easily and universally recognized between those 

 living things that are really cousins, or members of the same family. 



However, biologists nowadays claim that a closer examination of the un 

 explored and &quot;apparently bottomless&quot; rifts of cleavage, which mark off the 

 so-called &quot; natural kinds &quot; from one another, has brought to light two impor 

 tant facts: (i) that in very many cases these lines of demarcation are ex 

 ceedingly vague and difficult to trace, and (2) that there are evidences against 

 the internal fixity and external independence of many so-called &quot;species,&quot; 

 and in favour of the theory that these too are somewhat variable, and that 

 many of them have reached their present comparatively stable and differenti 

 ated condition by evolution from a common primitive stock. 



As regards the first of those two points, we have already recognized the 

 fact that, although there are innumerable clear and unmistakable &quot; differences &quot; 

 or &quot;dissimilarities,&quot; or grounds of division, between groups of objects both in 

 the organic and inorganic kingdoms of nature, nevertheless these lines are 

 oftentimes not hard and fast, -Natura non facit saltum, but such that the 

 groups seem to shade off imperceptibly into one another (53) ; so that in the 

 process of aggregating, or grouping individuals according to their resemblance 

 to some one or other of certain typical specimens, chosen as nuclei for the 

 formation of natural classes, we often reach a borderland of doubtful indi 

 viduals, any of which may apparently fall just as naturally into one as into 

 the other of two neighbouring classes : in which cases alternative classifica 

 tions will give rise to alternative definitions. 1 



68. How CLASSIFICATION MAY BE INFLUENCED BY HYPOTHESIS. 

 Such extreme or &quot; limiting &quot; examples as those just referred to will 

 serve as tests to determine which of the competing schemes of classifica- 



1 We have already called attention to the fact that when we speak of the Defini 

 tion of a thing as being per genus et differentiam we mean to refer to the genus and 

 differentia which it is found to possess in a natural scheme, not in any of the 

 possible artificial schemes, of classification (50, 51). Cf. VENN, op. cit., pp. 226, 

 336 ; JOSEPH, op. cit., pp. 85-92. 



