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The large sub-family of forms called "The Vertebrates" are distinguished from the Invertebrates by reason of the former possessing an internal bony skeleton, the most important feature of which is the vertebra or spinal column.
Class Aves. Order Incessores. Tribe Conirostres. Family Corvidae. Sub-family Corvinae. Genus Corvus. Sub-genus, or species Pica. This brings us down to species, the subdivision where intermarriage or breeding is usually considered as natural to animals, and where a resemblance of offspring to parents is generally persevered in.
The wolf and similar forms belong to the sub-family of dogs; while the lion, tiger, etc., belong to the sub-family of cats; The Rodentia, or Gnawers, comprising the rat, the hare, the beaver, the squirrel, the mouse, etc., etc.; The Insectivora, or Insect Feeders, comprising the mole, the shrew, the hedgehog, etc.;
If we imagine three lines of descent proceeding from a common stock, it is quite conceivable that two of them might after the lapse of ages be so slightly changed as still to remain as species of the same genus, whilst the third line might become so greatly modified as to deserve to rank as a distinct Sub-family, Family, or even Order.
Instances could be given amongst plants and insects, of a group of forms, 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 species, with slightly different grades of difference, have been subsequently discovered.
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