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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.