CHAPTER 13.  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.

From the first dawn of life, all organic beings are found to resemble each other in descending degrees, so that they can be classed in groups under groups.  This classification is evidently not arbitrary like the grouping of the stars in constellations.  The existence of groups would have been of simple signification, 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 in nature; for it is notorious how commonly members of even the same subgroup have different habits.  In our second and fourth chapters, on Variation and on Natural Selection, I have attempted to show that it is the widely ranging, the much diffused and common, that is the dominant species belonging to the larger genera, which vary most.  The varieties, or incipient species, thus produced ultimately become converted, as I believe, 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 indefinitely 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, there is a constant tendency in their characters to diverge.  This conclusion was supported by looking at the great diversity of the forms of life which, in any small area, come into the closest competition, and by looking to certain facts in naturalisation.

I attempted also to show that there is a constant tendency in the forms which are increasing in number and diverging in character, to supplant and exterminate the less divergent, the less improved, and preceding 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