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For example: “Swammerdam developed the extreme form of the preformation theory, supposing that an egg contained all the future generations of its kind as preformed miniatures, like a series of boxes one inside the other. ‘In nature there is no generation,’ wrote Swammerdam, ‘but only propagation, the growth of parts. Thus original sin is explained, for all men were contained in the organs of Adam and Eve.’” “Preformation” was a early theory of development, which flourished for over a century. It had its origins in one of Swammerdam’s most notorious experiments in which he dissected a silk-worm caterpillar and correctly showed the presence of adult structures such as wings, legs and antennae within its body, prior to pupation (this is true of all moths and butterflies). As early as 1665, in front of a meeting of Thévenot’s circle (a predecessor of the French Académie des Sciences), he dissected out these adult structures, showing “a Butterfly enclosed and hidden in a caterpillar, and perfectly contained within its skin” as he put it, much to the amazement of his audience (and indeed, to some modern commentators who have refused to believe what he found). Swammerdam drew two conclusions from this: first, that the caterpillar and the adult were the same individual in different forms (in itself this was a revolutionary statement), and second that adult forms lie hidden within the juvenile body. This is what he meant by the statement “in the whole of nature there is no generation, only growth”. This was opposed to the vague Aristotelian notion of “epigenesis” put forward by William Harvey in 1651, according to which development was the “addition of parts that successively arise” as the result of a “contagion [...] communicated in coitu by the father”. In the early 1670s, the Cartesian priest Malebranche popularised Swammerdam’s view by combining it with the mathematical idea of infinity recently developed by Pascal and the fact that, seen under the microscope, life-forms indeed appeared to recede into the infinitely small. Malebranche’s conclusion —accepted by Swammerdam in the pages of his book Miraculum Naturae and in the quote above — was that each seed or egg contains within it all its descendants, in a form of “emboîtement”. But Swammerdam did not argue that the whole adult butterfly was literally present under the caterpillar’s cuticle (he was far too skilled an observer to make such a mistake). Ahe admitted in The Book of Nature, these structures are partial, extremely fragile and can only be seen in caterpillars that are close to pupation. His objective was simply to demonstrate what today seems obvious: the same individual is present in all stages of the insect life-cycle. The proof that he had no conception of “emboîtement” is shown by his study of the “Fourth Order” (mainly flies), where metamorphosis involves the virtually complete dissolution of the larval structures and, unlike the butterfly, no adult structures can be seen prior to pupation. Far from arguing that the forms of the adult fly are present within larva, Swammerdam describes the massive changes that occur in the nervous and digestive systems and the muscles of the larva during pupation. On the basis of a series of dissections at different stages in development, he rightly insists that there is no “metamorphosis” in the sense of one individual changing into another, but rather that the same individual undergoes “astonishing transformations” and “disagreeing transmutations”. In both cases — butterflies and flies — Swammerdam was right. In a bold step that laid the basis of a materialist understanding of development, he showed that there was no spontaneous generation and that the same organism was present in egg, larva, pupa and adult. He did not put forward a theory of “preformation”. Indeed, he was reluctant to speculate in any way, preferring to rely on the results of his (generally accurate) observations. As to his remarks about the eggs of Adam and original sin, two points need to be made. Firstly, this was based in part on his striking observation that eggs could be seen in the ovaries of unborn female mammals. Given that life apparently receded beyond the resolution of the microscope (for example, he found worms within the worms he observed inside the snail’s uterus), it must have seemed possible that eggs might regress back to the origin of the world. This did not mean, however, that Swammerdam thought that these were fully-formed individuals one within another. Second, in the whole of The Book of Nature, Swammerdam makes only two passing remarks relating his view of “preformation” to his religious convictions. Far from forming the thrust of his work, this particular combination of science and theology is an insignificant (but interesting) detour that should not be taken as characteristic of his work. The real development of preformationism took place after his death. In the early years of the 20th century, F. J. Cole and Joseph Needham made the first serious attempts to study the history of biology. Both were hostile to preformationism: Cole argued that it was “a deadly obstruction” that had “stopped the clock” of scientific progress through its “long and evil reign”, while Needham thought it possessed a “cloven hoof”. This was not only unfair to Swammerdam, it was unfair to preformationism. In terms of generating further discoveries — one of the most important measures of the value of a scientific theory — preformationism turned out to be remarkably prolific. It was rapidly accepted, although not always in the same form — preformationists became divided into “spermists” and “ovists”, depending on whether they thought sperm or eggs were decisive for the appearance of the future individual. Throughout the 18th century preformationism was the standard against which scientists measured their discoveries relating to generation, and only began to collapse when it was found that some organisms, such as polyps, could regenerate from isolated parts rather than from an egg or sperm. This page is partly excerpted and adapted from an article that appeared in Endeavour (2000) 24(3) pp122-128. © Elsevier Science, 2000. To download a PDF version of the full article, click here. |