Wonderful Life:The Burgess Shale and the Nature of History(97)By: Stephen Jay Gould
The infamous “Lipalian interval” was Walcott’s name for this time of Precambrian nondeposition. Walcott proposed a world-wide break in accessible marine sedimentation, just during the critical interval of extensive Precambrian ancestry for modern groups. In a famous address to the Eleventh International Geological Congress, meeting in Stockholm on August 18,1910, he stated:
I have for the past 18 years watched the geological and paleontological evidence that might aid in solving the problem of Precambrian life. The great series of Cambrian and Precambrian strata in eastern North America from Alabama to Labrador; in western North America from Nevada and California far into Alberta and British Columbia, and also in China, have been studied and searched for evidences of life until the conclusion had gradually been forced upon one that on the North American continent we have no known Precambrian marine deposits containing traces of organic remains, and that the abrupt appearance of the Cambrian fauna results from geological and not from biotic conditions.… In a word, the thought is that the Algonkian [late Precambrian] period … was a period of continental elevation and largely terrigenous sedimentation in non-marine bodies of water, also a period of deposition by aerial and stream processes over considerable areas (1910, pp. 2–4).
And he added:
Lipalian is proposed for the era of unknown marine sedimentation.… The apparently abrupt appearance of the lower Cambrian fauna is … to be explained by the absence near our present land area of the sediments, and hence the faunas of the Lipalian period (1910, p. 14).
Walcott’s explanation may sound forced and ad hoc. It was surely born of frustration, rather than the pleasure of discovery. Yet the nonexistent Lipalian was not a fool’s rationalization, as usually presented in our textbooks, but a credible synthesis of geological evidence in the context of a vexatious dilemma. If Walcott deserves any brickbats, direct them at his failure to consider any alternative to his favored way of thinking about the artifact theory—and at his false assumption, imposed by the old bias of gradualism, that equated evolution itself with a long sequence of ancestral continuity for any complex creature. For even if the Lipalian hypothesis made sense in the light of existing geological information, it rested, as Walcott knew only too well, upon the most treacherous kind of argument that a scientist can ever use—negative evidence. Walcott admitted: “I fully realize that the conclusions above outlined are based primarily on the absence of a marine fauna in Algonkian rocks” (1910, p. 6).
And, as so often happens in the face of negative evidence, the earth eventually responded, offering to later geologists abundant late Precambrian marine sediments—still with no fossils of complex invertebrates. The Lipalian interval ended up on the trash heap of history.
Scientists have a favorite term for describing a phenomenon like Walcott’s allegiance to the Burgess shoehorn—overdetermined. The modern concept of maximal disparity and later decimation (perhaps by lottery) never had the ghost of a chance with Walcott because so many elements of his life and soul conspired to guarantee the opposite view of the shoehorn. Any one of these elements would have been enough in itself; together, they overwhelmed any alternative, and overdetermined Walcott’s interpretation of his greatest discovery.
To begin, as we have seen, Walcott’s persona as an archtraditionalist in thought and practice did not lead him to favor unconventional interpretations in any area of life. His general attitude to life’s history and evolution implied stately unfolding along predictable pathways defined by the ladder of progress and cone of increasing diversity; this pattern also held moral meaning, as a display of God’s intention to imbue life with consciousness after a long history of upward striving. Walcott’s specific approach to the key problem that had focused his entire career—the riddle of the Cambrian explosion—favored a small set of stable and well-separated groups during Burgess times, so that a long history of Precambrian life might be affirmed, and the artifact theory of the Cambrian explosion supported. Finally, if Walcott had been at all inclined to abandon his ideological commitment to the shoehorn, in the light of contradictory data from the Burgess Shale, his administrative burdens would not have allowed him time to study the Burgess fossils with anything like the requisite care and attention.
I have labored through the details of Walcott’s interpretation and its sources because I know no finer illustration of the most important message taught by the history of science: the subtle and inevitable hold that theory exerts upon data and observation. Reality does not speak to us objectively, and no scientist can be free from constraints of psyche and society. The greatest impediment to scientific innovation is usually a conceptual lock, not a factual lack.