So Simple a Beginning

*In the sixth edition Darwin wielded the word evolution — and “evolutionist”, referring presumably to someone whose systematic understanding of the species problem extended beyond natural history, a scientific thinker, and not only an observer. He did so in response to a now-familiar criticism of evolutionary ideas, that natural selection could not produce a complex result out of rudimentary and presumably useless (or even harmful) structures. How could you get an eye, Darwin’s critics asked most famously, out of rudimentary, pieces, or of other examples to which Darwin responds to in the sixth edition. Darwin’s answer is the same as the one modern biologists have worked out in exquisite detail in the matter of the evolution of the eye: look around you and you will find examples of advantageous intermediate forms of complicated biological structures throughout nature. It was never wise to challenge Charles Darwin to deploy facts of natural history in support of his theory, and his critic St. George Jackson Mivart found himself on the receiving end of a howitzer’s worth of information, delivered, as ever, in Darwin’s usual, understated voice. For instance, consider the starfish:

The Echinodermata (star-fishes, sea-urchins, &c.) are furnished with remarkable organs, called pedicellariæ, which consist, when well developed, of a tridactyle forceps—that is, of one formed of three serrated arms, neatly fitting together and placed on the summit of a flexible stem, moved by muscles. These forceps can seize firmly hold of any object; and Alexander Agassiz has seen an Echinus or sea-urchin rapidly passing particles of excrement from forceps to forceps down certain lines of its body, in order that its shell should not be fouled. But there is no doubt that besides removing dirt of all kinds, they subserve other functions; and one of these apparently is defence.

With respect to these organs, Mr. Mivart, as on so many previous occasions, asks: “What would be the utility of the first rudimentary beginnings of such structures, and how could such incipient buddings have ever preserved the life of a single Echinus?” He adds, “Not even the sudden development of the snapping action could have been beneficial without the freely moveable stalk, nor could the latter have been efficient without the snapping jaws, yet no minute merely indefinite variations could simultaneously evolve these complex co-ordinations of structure; to deny this seems to do no less than to affirm a startling paradox.” Paradoxical as this may appear to Mr. Mivart, tridactyle forcepses, immovably fixed at the base, but capable of a snapping action, certainly exist on some star-fishes; and this is intelligible if they serve, at least in part, as a means of defence. Mr. Agassiz, to whose great kindness I am indebted for much information on the subject, informs me that there are other star-fishes, in which one of the three arms of the forceps is reduced to a support for the other two; and again, other genera in which the third arm is completely lost. In Echinoneus, the shell is described by M. Perrier as bearing two kinds of pedicellariæ, one resembling those of Echinus, and the other those of Spatangus; and such cases are always interesting as affording the means of apparently sudden transitions, through the abortion of one of the two states of an organ.

With respect to the steps by which these curious organs have been evolved, Mr. Agassiz infers from his own researches and those of Müller, that both in star-fishes and sea-urchins the pedicellariæ must undoubtedly be looked at as modified spines. This may be inferred from their manner of development in the individual, as well as from a long and perfect series of gradations in different species and genera, from simple granules to ordinary spines, to perfect tridactyle pedicellariæ. The gradation extends even to the manner in which ordinary spines and the pedicellariæ with their supporting calcareous rods are articulated to the shell. In certain genera of star-fishes, “the very combinations needed to show that the pedicellariæ are only modified branching spines” may be found. Thus we have fixed spines, with three equi-distant, serrated, moveable branches, articulated to near their bases; and higher up, on the same spine, three other moveable branches. Now when the latter arise from the summit of a spine they form in fact a rude tridactyle pedicellaria, and such may be seen on the same spine together with the three lower branches. In this case the identity in nature between the arms of the pedicellariæ and the moveable branches of a spine, is unmistakable. It is generally admitted that the ordinary spines serve as a protection; and if so, there can be no reason to doubt that those furnished with serrated and moveable branches likewise serve for the same purpose; and they would thus serve still more effectively as soon as by meeting together they acted as a prehensile or snapping apparatus. Thus every gradation, from an ordinary fixed spine to a fixed pedicellaria, would be of service.

And, just to deal with the more notorious question of what good is an imperfect eye, such that evolution by natural selection could act upon it to produce more perfect organs, see see Russell Fernald’s review in Science, 29 September 2006 Vol 313, pp. 1914-1918.