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13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Or 13-03 - Rules and difficulties in classification, explained on the theory of descent with modification 20 These resemblances, though so intimately connected with the whole life of the being, are ranked as merely `adaptive or analogical characters;' but to the consideration of these resemblances we shall have to recur.

It may even be given as a general rule, that the less any part of the organisation is concerned with special habits, the more important it becomes for classification.

As an instance: Owen, in speaking of the dugong, says, `The generative organs being those which are most remotely related to the habits and food of an animal, I have always regarded as affording very clear indications of its true affinities.

Richard Owen
Richard Owen


We are least likely in the modifications of these organs to mistake a merely adaptive for an essential character.' So with plants, how remarkable it is that the organs of vegetation, on which their whole life depends, are of little signification, excepting in the first main divisions; whereas the organs of reproduction, with their product the seed, are of paramount importance!

We must not, therefore, in classifying, trust to resemblances in parts of the organisation, however important they may be for the welfare of the being in relation to the outer world.
13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Or 13-06 - Analogical or adaptive characters 20 In the chapter on geological succession I attempted to show, on the principle of each group having generally diverged much in character during the long-continued process of modification, how it is that the more ancient forms of life often present characters in some slight degree intermediate between existing groups.

A few old and intermediate parent-forms having occasionally transmitted to the present day descendants but little modified, will give to us our so-called osculant or aberrant groups.

The more aberrant any form is, the greater must be the number of connecting forms which on my theory have been exterminated and utterly lost.

And we have some evidence of aberrant forms having suffered severely from extinction, for they are generally represented by extremely few species; and such species as do occur are generally very distinct from each other, which again implies extinction.

The genera Ornithorhynchus and Lepidosiren, for example, would not have been less aberrant had each been represented by a dozen species instead of by a single one; but such richness in species, as I find after some investigation, does not commonly fall to the lot of aberrant genera.

ornithorhynchus
ornithorhynchus

lepidosiren
lepidosiren


We can, I think, account for this fact only by looking at aberrant forms as failing groups conquered by more successful competitors, with a few members preserved by some unusual coincidence of favourable circumstances.
13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Or 13-07 - Affinities, general, complex and radiating 20 The elder De Candolle has made nearly similar observations on the general nature of the affinities of distinct orders of plants.

Alphonse de Candolle
Alphonse de Candolle


On the principle of the multiplication and gradual divergence in character of the species descended from a common parent, together with their retention by inheritance of some characters in common, we can understand the excessively complex and radiating affinities by which all the members of the same family or higher group are connected together.

For the common parent of a whole family of species, now broken up by extinction into distinct groups and sub-groups, will have transmitted some of its characters, modified in various ways and degrees, to all; and the several species will consequently be related to each other by circuitous lines of affinity of various lengths (as may be seen in the diagram so often referred to), mounting up through many predecessors.

As it is difficult to show the blood-relationship between the numerous kindred of any ancient and noble family, even by the aid of a genealogical tree, and almost impossible to do this without this aid, we can understand the extraordinary difficulty which naturalists have experienced in describing, without the aid of a diagram, the various affinities which they perceive between the many living and extinct members of the same great natural class.
13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Or 13-08 - Extinction separates and defines groups 20 There has been less entire extinction of the forms of life which once connected fishes with batrachians.

fish
fish


There has been still less in some other classes, as in that of the Crustacea, for here the most wonderfully diverse forms are still tied together by a long, but broken, chain of affinities.

crustacean
crustacean