Lazzaro Spallanzani’s Scientific “Heresy”
by Guy Lyon Playfair
In 1794 the eminent Italian physiologist Lazzaro Spallanzani (1729-99), one of the founders of experimental biology, published a modest but heretical proposal.
Long intrigued by the ability of bats to fly in total darkness without bumping into things, he set out to discover how they did it. He reasoned that they must be using one of their five senses, and in a series of extremely cruel experiments he maimed bats by destroying their senses one by one, blinding them, blocking their ears or even cutting them off, eliminating their sense of smell and removing their tongues.
It soon became clear to him that it was the sense of hearing that bats needed in order to avoid obstacles. But hearing what? Bats made no audible sounds as they flew, and little if anything was known in the 18th century about ultrasound, the secret of bats’ success as nocturnal navigators. As they fly, they emit beams of up to 50,000 cycles per second – more than twice the upper limit of human hearing – and ‘read’ the returning echoes. It was a striking example, of which there are many, of a man-made invention, in this case echo location or sonar, having existed in nature long before we reinvented it.
Spallanzani was in effect making a claim for the paranormal, much as the pioneers of psychical research were to do in the following century in the case of telepathy. There was no sign in 1794 of a normal explanation for the bat’s navigating skills, so the scientific establishment did what it tends to do on these occasions – it made one up. Its chief spokesman was the French naturalist Georges Cuvier (1769-1832), a pioneer in both anatomy and palaeontology. He decreed, in a paper published in 1795, that “to us, the organs of touch seem sufficient to explain all the phenomena which bats exhibit”.
He had it all worked out. Bats’ wings were “richly supplied with nerves of every type”, which could somehow or other receive impressions of heat, cold and resistance. Yet whereas Spallanzani, and several colleagues whom he persuaded to repeat his experiments, reached their unanimous conclusion only after numerous experiments, Cuvier solved the problem without having performed a single one. It was, as the 20th century bat expert Robert Galambos noted, “a triumph of logic over experimentation”.
It was also a triumph of ignorance over knowledge. One of Spallanzani’s colleagues had actually thought of the sensitive-wing theory and tested it, by putting bats in an all-white room and coating their wingtips with some kind of black stuff that would come off on the walls and various white objects if the bats’ wings touched them. They didn’t.
Cuvier’s explanation soon found its way into the textbooks, and stayed there until the start of the 20th century, when independent researchers in France and the USA published yet more experimental evidence in support of Spallanzani’s theory. Then, in 1920, a British researcher named Hartridge who had helped to develop the first naval sonar systems during World War 1, published the first clearly stated theory of bat navigation by ultrasound. This was duly confirmed, using newly developed recording devices, by Galambos and his colleague Donald Griffin, who published their results in 1941 – nearly a century and a half after Spallanzani.
In retrospect, it is hard to see how those original findings took so long to gain acceptance. Spallanzani was no maverick amateur, but a versatile and experienced researcher regarded as one of the leading physiologists of his day who did pioneering work in such areas as fertilisation, artificial insemination and limb regeneration. In his bat research he followed what is now the normal practice of inviting colleagues to replicate one’s findings or claims. His work was widely disseminated – an English translation of the final and fairly conclusive report of his Swiss collaborator Louis Jurine appeared in the first volume (1798) of the Philosophical Magazine. Above all. the acoustic theory was solidly based on the experimental evidence of several independent researchers. Yet it was to remain neglected for more than a century largely thanks to the immense prestige of Cuvier, whom Napoleon put in charge of French educational reform. Lone voices of dissent, such as that of British physician Sir Anthony Carlisle, who concluded, after carrying out his own experiments, that bats avoided obstacles “owing to extreme acuteness of hearing” went largely unheard. A more typical attitude was expressed in 1809 by one George Montagu, who asked sarcastically “Since bats see with their ears, do they hear with their eyes?”
“Had he [Spallanzani] been taken seriously, how much sooner might we have discovered radar?” asked the late Eric Laithwaite, an engineer with a keen interest in natural technology. It would only have to have been invented five or ten years earlier to have possibly saved the more than 1,500 lives lost when the Titanic hit an iceberg in 1912. Bats do not fly into icebergs or anything else, and it should have been possible to work out how long before it finally was. Laithwaite added: “Trying to discover how a biological mechanism works has an advantage over solving problems in non-biological areas since one is sure the problem can be solved.” Since Nature has already solved her problems, the researcher has the sure knowledge that a solution exists.
However, as long as the spirit of the Cuviers of this world lives on, as it still does in such organisations as CSICOP [now CSI], many of them may remain unsolved for another century or so.
Thought for the Day
“In moderate doses, skepticism stimulates the search for truth; in immoderate doses, it inhibits such a search.”
– Mario Bunge, The Skeptical Inquirer, July/Aug. 2000, p. 6
Galambos, R. (1942) The avoidance of obstacles by flying bats. Isis 34, 132-40.
Hartridge, N. (1920) The avoidance of objects by bats in their flight. Journal of Physiology 54, 54-7.
Laithwaite, E. R. (1977) Biological analogues in engineering practice. Interdisciplinary Science Reviews 2(2), 100-8.