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On the Origin of Species

Mutual Affinities Of Organic Beings: Morphology --
Embryology -- Rudimentary Organs
Classification, groups subordinate to groups -- Natural system -- Rules and difficulties in
classification, explained on the theory of descent with modification -- Classification of
varieties -- Descent always used in classification -- Analogical or adaptive characters --
Affinities, general, complex and radiating -- Extinction separates and defines groups --
Morphology, between members of the same class, between parts of the same individual --
Embryology, laws of, explained by variations not supervening at an early age, and being
inherited at a corresponding age -- Rudimentary organs; their origin explained --
Summary.
CLASSIFICATION.
From the most remote period in the history of the world organic beings have been found
to resemble each other in descending degrees, so that they can be classed in groups under
groups. This classification is not arbitrary like the grouping of the stars in constellations.
The existence of groups would have been of simple significance, if one group had been
exclusively fitted to inhabit the land, and another the water; one to feed on flesh, another
on vegetable matter, and so on; but the case is widely different, for it is notorious how
commonly members of even the same subgroup have different habits. In the second and
fourth chapters, on Variation and on Natural Selection, I have attempted to show that
within each country it is the widely ranging, the much diffused and common, that is the
dominant species, belonging to the larger genera in each class, which vary most. The
varieties, or incipient species, thus produced, ultimately become converted into new and
distinct species; and these, on the principle of inheritance, tend to produce other new and
dominant species. Consequently the groups which are now large, and which generally
include many dominant species, tend to go on increasing in size. I further attempted to
show that from the varying descendants of each species trying to occupy as many and as
different places as possible in the economy of nature, they constantly tend to diverge in
character. This latter conclusion is supported by observing the great diversity of forms,
which, in any small area, come into the closest competition, and by certain facts in
naturalisation.
I attempted also to show that there is a steady tendency in the forms which are increasing
in number and diverging in character, to supplant and exterminate the preceding, less
divergent and less improved forms. I request the reader to turn to the diagram illustrating
the action, as formerly explained, of these several principles; and he will see that the
inevitable result is, that the modified descendants proceeding from one progenitor
become broken up into groups subordinate to groups. In the diagram each letter on the
uppermost line may represent a genus including several species; and the whole of the
genera along this upper line form together one class, for all are descended from one
ancient parent, and, consequently, have inherited something in common. But the three
genera on the left hand have, on this same principle, much in common, and form a
subfamily, distinct from that containing the next two genera on the right hand, which
 
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