304 HOUSE, GARDEN, AND FIELD 



For nearly two centuries it has-been admitted that 

 plants and animals must be classified according to their 

 natural affinities. Nobody however could be got to 

 explain what he meant by affinity. They talked much 

 about affinity, and they really recognised it, but they 

 could not say what it was. Every fresh systematist pro- 

 posed his arrangement, which was praised as natural or 

 blamed as unnatural, and in time opinion became fixed 

 as to the primary groups at least, although no logical 

 basis of a natural classification had yet been discovered. 

 In one important respect the accepted systems infringed 

 a universally admitted logical rule. Every one agrees 

 that whatever property is selected as the basis of an 

 arrangement, it must be kept to throughout. In classi- 

 fying books you may go upon subject, or size, or alpha- 

 betical order of authors' names, but if you begin with one 

 of these and afterwards change to another, you will get 

 into hopeless confusion. Now this was just what the 

 naturalists did, or seemed to do. Indeed they discovered 

 that the classifications which best satisfied their sense of 

 affinity continually changed their basis. All classifica- 

 tions by characters taken from single organs, corolla, 

 stamens, organs of circulation, organs of respiration, or 

 whatever it might be, proved unsatisfactory. The in- 

 creasing unanimity of naturalists on fundamental points 

 showed, however, that whether they conformed to the 

 rules of logic or not, they were in all probability making a 

 nearer and nearer approach to scientific truth. 



Such was the state of matters fifty years ago, when 

 Darwin put forth his doctrine of the Origin of Species, 

 which threw a flood of light upon the classification of plants 

 and animals. Darwin gave reasons for believing that 

 animals and plants now quite distinct from one another 

 have often descended from a common ancestor. Affinity 

 he interpreted literally, as the result of common descent ; 

 natural groups are collections of species whose likeness to 

 one another is derived from common descent, and their 



