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14 - Recapitulation and Conclusion 14-04 - How far the theory of natural selection may be extended 10 It may be asked how far I extend the doctrine of the modification of species.

The question is difficult to answer, because the more distinct the forms are which we may consider, by so much the arguments fall away in force.

But some arguments of the greatest weight extend very far.

All the members of whole classes can be connected together by chains of affinities, and all can be classified on the same principle, in groups subordinate to groups.

Fossil remains sometimes tend to fill up very wide intervals between existing orders.

Organs in a rudimentary condition plainly show that an early progenitor had the organ in a fully developed state; and this in some instances necessarily implies an enormous amount of modification in the descendants.

Throughout whole classes various structures are formed on the same pattern, and at an embryonic age the species closely resemble each other.

Therefore I cannot doubt that the theory of descent with modification embraces all the members of the same class.

I believe that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number.

Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype.

But analogy may be a deceitful guide.

Nevertheless all living things have much in common, in their chemical composition, their germinal vesicles, their cellular structure, and their laws of growth and reproduction.

We see this even in so trifling a circumstance as that the same poison often similarly affects plants and animals; or that the poison secreted by the gall-fly produces monstrous growths on the wild rose or oak-tree.

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rose

Gall Wasp
Gall Wasp

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oak


Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.