Nevertheless, when folds growth is prolonged by evolution, it is so in the direction of the existing fold. Stated otherwise, if the appearance of a fold corresponds to crossing of a generic biomechanical threshold for folding, one may expect that the fold is itself stable in an open set of parameters, such that, “conserving the scaffold”, as stated by Darwin, a fold may elongate or shorten during evolution. This is to say that the more or less elongated aspect of rays in ﬁns is somewhat predictable, it suﬃces to push forward the pattern of ﬁsh bones in the rays. This is also true of other edge-like organs, such as ears, nostrils or lips, which are well known to be observed in nature with almost “arbitrary” lengths, a word often used by Darwin.
“Arbitrary” is found in five occurrences in the 1859 edition of On the Origin of Species, referenced by Fleury (and in the other five editions as you may check).
None of the occurrences have anything to do with “arbitrary” lengths.
The frequency of the word “arbitrary” is infinitely greater then the one of the world “scaffold” which is completely absent. But lower then that of the word “pigeon” (121 occurrences) which Fleury may think his readers are (at least the intended audience, physicist, that’s it).
(p48 DOUBTFUL SPECIES. CHAP. II.): Many years ago, when comparing, and seeing others compare, the birds from the separate islands of the Galapagos Archipelago, both one with another, and with those from the American mainland, I was much struck how entirely vague and arbitrary is the distinction between species and varieties.
(p411 CHAP. XIII. CLASSIFICATION.): 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.
(p419 CHAP. XIII. CLASSIFICATION.): Finally, with respect to the comparative value of the various groups of species, such as orders, sub-orders, families, sub-families, and genera, they seem to be, at least at present, almost arbitrary. Several of the best botanists, such as Mr. Bentham and others, have strongly insisted on their arbitrary value. Instances could be given amongst plants and insects, of a group of forms, first ranked by practised naturalists as only a genus, and then raised to the rank of a sub-family or family; and this has been done, not because further research has detected important structural differences, at first overlooked, but because numerous allied species, with slightly different grades of difference, have been subsequently discovered.
(p428 CLASSIFICATION. CHAP. XIII.)As members of distinct classes have often been adapted by successive slight modifications to live under nearly similar circumstances,—to inhabit for instance the three elements of land, air, and water,—we can perhaps understand how it is that a numerical parallelism has sometimes been observed between the sub-groups in distinct classes. A naturalist, struck by a parallelism of this nature in any one class, by arbitrarily raising or sinking the value of the groups in other classes (and all our experience shows that this valuation has hitherto been arbitrary), could easily extend the parallelism over a wide range; and thus the septenary, quinary, quaternary, and ternary classifications have probably arisen.
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