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 
