We are led to the same conclusion, namely, the independence of parts successively developed, by another and quite distinct group of facts. It is well known that many animals belonging to the same class, and therefore not differing widely from each other, pass through an extremely different course of development. Thus certain beetles, not in any way remarkably different from others of the same order, undergo what has been called a hyper-metamorphosis—that is, they pass through an early stage wholly different from the ordinary grub-like larva. In the same sub-order of crabs, namely, the Macroura, as Fritz Müller remarks, the river cray-fish is hatched under the same form which it ever afterwards retains; the young lobster has divided legs, like a Mysis; the Palæmon appears under the form of a Zoea, and Peneus under the Nauplius-form; and how wonderfully these larval forms differ from each other, is known to every naturalist. Some other crustaceans, as the same author observes, start from the same point and arrive at nearly the same end, but in the middle of their development are widely different from each other.
In other words, Darwin has twigged to the important idea that each stage of the life-cycle of an organisms is under different selective pressures, and thus one stage can diverge while the others remain quite similar.
Crayfish are also mentioned in one of Darwin’s transmutation notebooks. In it, Darwin shows his admiration for the natural world (“beautiful adaptations”), and interest in explaining the non-adaptive features (“unintelligible structures”):
The question if creative power acted at Galapagos it so acted that birds with plumage and tone of voice purely American, North and South; so permanent a breath cannot reside in space before island existed. Such an influence must exist in such spots. We know birds do arrive, and seeds. (And geographical divisions are arbitrary and not permanent. This might be made very strong, if we believe the Creator created by any laws, which I think is shown by the very facts of the Zoological character of these islands.)
The same remarks applicable to fossil animals same type; armadillo-like cray [i.e., crayfish?] created; passage for vertebrae in neck same cause. Such beautiful adaptations, yet other animals live so well. This view of propagation gives a hiding place for many unintelligible structures; it might have been of use in progenitor, or it may be of use,—like mammae on man's breast.
In the Voyage of the Beagle, crayfish feature in a segment that shows the less enlightened aspect of Darwin’s cultural background, with the sort of references to “savages” that interested colonial powers.
The little stream, besides its cool water, produced eels and cray-fish. I did indeed admire this scene, when I compared it with an uncultivated one in the temperate zones. I felt the force of the observation, that man, at least savage man, with his reasoning powers only partly developed, is the child of the tropics.
More on Darwin’s ties to crustaceans can be found in today’s post on NeuroDojo.
