Classification. 43 



differs from that of an astronomer, in that the 

 objects which he has to classify present structural 

 resemblances and structural differences in numberless 

 degrees ; and it is the object of his classification to 

 present a tabular statement of these facts. Now, 

 long before the theory of evolution was entertained, 

 naturalists became fully aware that these facts of 

 structural resemblances running through groups sub- 

 ordinate to groups were really facts of nature, and 

 not merely poetic imaginations of the mind. No one 

 could dissect a number of fishes without perceiving 

 that they were all constructed on one anatomical 

 pattern, which differed considerably from the equally 

 uniform pattern on which all mammals were con- 

 structed, even although some mammals bore an 

 extraordinary resemblance to fish in external form 

 and habits of life. And similarly with all the smaller 

 divisions of the animal and vegetable kingdoms. 

 Everywhere investigation revealed the bonds of close 

 structural resemblances between species of the same 

 genus, resemblance less close between genera of the 

 same family, resemblance still less close between 

 families of the same order, resemblance yet more 

 remote between orders of the same class, and resem- 

 blance only in fundamental features between classes 

 of the same sub-kingdom, beyond which limit all 

 anatomical resemblance was found to disappear 

 the different sub-kingdoms being formed on wholly 

 different patterns. Furthermore, in tracing all these 

 grades of structural relationship, naturalists were 

 slowly led to recognise that the form which a natural 

 classification must eventually assume would be that 

 of a tree, wherein the constituent branches would 



