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definition has hitherto been found impossible. There are crustaceans at the opposite ends of the series, which have hardly a character in common; yet the species at both ends, from being plainly allied to others, and these to others, and so onwards, can be recognised as unequivocally belonging to this, and to no other class of the Articulata. |
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| Geographical distribution has often been used, though perhaps not quite logically, in classification, more especially in very large groups of closely allied forms. Temminck insists on the utility or even necessity of this practice in certain groups of birds; and it has been followed by several entomologists and botanists. |
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| Finally, with respect to the comparative value of the various groups of species, such as orders, sub-orders, families, sub-families, and genera, they seem to be, at least at present, almost arbitrary. Several of the best botanists, such as Mr. Bentham and others, have strongly insisted on their arbitrary value. Instances could be given amongst plants and insects, of a group
first ranked by practised naturalists as only a genus, and then raised to the rank of a sub-family or family; and this has been done, not because further research has detected important structural differences, at first overlooked, but because numerous allied
with slightly different grades of difference, have been subsequently discovered. |
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| All the foregoing rules and aids and difficulties in classification
explained, if I do not greatly deceive myself, on the view that the
is founded on descent with
that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent,
→OMIT
all true classification
that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike. |
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| But I must explain my meaning more fully. I believe that the
arrangement
of the groups within each class, in due subordination and relation to
must be strictly genealogical in order to be natural; but that the
amount
of difference in the several branches or groups, though allied in the same degree in blood to their common progenitor, may differ greatly, being due to the different degrees of modification which they have undergone; and this is expressed by the forms being ranked under different genera, families, sections, or orders. The reader will best understand what is meant, if he will take the trouble
to the diagram in the fourth chapter. We will suppose the letters A to L to represent
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