Eight Little Piggies (3 page)

In 1906, after reading Gulick’s monograph, Henry Edward Crampton decided to enter the fray and to devote the remaining fifty years of his career to an immense study of
Partula
on Tahiti, Moorea, and surrounding islands. Crampton (1875–1956) had done excellent work in experimental embryology and studies of natural selection. This earlier effort led to a slight preference for adaptation, but Crampton maintained an open mind and was prepared to support Gulick’s “internal factors” against Wallace’s shaping by environment should the evidence warrant. Crampton made twelve expeditions to the Pacific and published three magnificent monographs—probably the finest work ever done on the evolution of land snails—collectively entitled
Studies on the Variation, Distribution, and Evolution of the Genus
Partula (Tahiti in 1917, other islands in 1925, and Moorea in 1932).

In short, and to summarize a half century of effort in a sentence, Crampton came down firmly on Gulick’s side. He could find no evidence that the forms and colors of
Partula
could be predicted from surrounding environments. Identical climatic conditions seemed to evoke different solutions time after time.

Crampton interpreted the differences between snails in adjacent valleys as results of three major causes—isolation, mutation (“congenital factors” in his terminology), and adaptation by natural selection—with only a minor role for Darwin’s favorite mechanism. He viewed the first factor, isolation, as a disposing precondition rather than an actual cause: Geographic separation produces nothing directly but establishes an independent population in which new features may spread. He saw the third factor, natural selection, as primarily negative. Once new features arise by some other mechanism, natural selection may eliminate them if they prove unworkable—but the source of creative change must lie elsewhere. Crampton, who was one of the first American biologists to recognize the importance of Mendel’s work, located this source of creativity in his second factor of mutation, or “internally generated” change by congenital factors. In any environment, hundreds of possible anatomies might work—and the forms and colors of this particular population in that specific valley are fortuitous consequences of the largely nonadaptive mutations that happened to arise and spread in an isolated population.

The resulting pattern of differences among valleys is largely nonadaptive. Every local race must avoid elimination by natural selection (and is fit in this negative sense), but its particular features represent only one in a myriad of workable possibilities, and any particular solution arises by the happenstance of mutation in an isolated population, not by natural selection. Crampton contrasted the greater importance of mutation over selection in writing about
Partula
on Tahiti:

How can we assess the importance of Crampton’s work sixty years after his last great monograph on
Partula
from Moorea (1932)? I am biased to be sure, for snail men (I am one) revere Crampton as a kind of patron saint, but I rank Crampton’s
Partula
studies among the most important in the history of evolutionary biology for three major reasons. First, he was probably right in his central claim about the nonadaptive nature of most small-scale differences in form and color among snails of adjacent valleys. Evolutionary biology went through a phase of strong belief in strict adaptationism in the generation just following Crampton, and his works did suffer a temporary eclipse. But his three great monographs are winning new respect and attention in our current, more pluralistic climate of opinion.

Second, Crampton must gain our highest admiration, verging on appropriate awe, for the sheer dedication and effort of his immense labors. I spent only a day in a rented car on Moorea, and scarcely ventured out of the shade or off the paths—but I still nearly passed out from sunstroke. Crampton spent months on twelve separate expeditions, all in an age of ships and horses (not to mention shank’s mare), rather than airplanes and rent-a-cars. In the charming understatement of conventional “objective” scientific prose, Crampton wrote but one small comment on working conditions:

But Crampton also recorded the countervailing pleasures that keep us all going:

Moreover, Crampton’s labor only began with collecting. He then spent years measuring his snails (some 80,000 for the Tahiti monograph, and a whopping 116,000 for the Moorea work) and calculating statistics—all of which, incredibly (even for his day), he did personally and by hand! (No computers, no hand-held calculators; when Crampton speaks of “calculating machines,” he means those old mechanical jobbies that performed division by successive subtraction and clanked away for minutes to perform simple operations.) Again, he wrote in understatement:

Third, and most important, ultimate judgment must reside in a criterion of utility. All good science is accumulative; no one can get everything right the first time. If Crampton’s monographs were only monuments to past effort and ideas, they might still be admired, but only as items of human paleontology. They are, in fact, precious mines for continuing revision and extension. I know this in the most personal way, for I have used Crampton’s tables, the product of his years of “mathematical drudgery,” in at least three of my technical papers.

A figure from Crampton’s monograph illustrating a
Partula
shell and some of the measurements that he made on each specimen.
Carnegie Institution of Washington
.

To put this crucial point in another and stronger way, Crampton spent fifty years documenting the
current
geographic distribution and variation of
Partula
on Tahiti, Moorea, and nearby islands. This work has great and permanent value as a frozen snapshot, but Crampton’s half century should be but a transient moment in the future history of
Partula
. Crampton devoted this lifetime of effort in order to
establish a baseline for future work. Partula
would continue to evolve rapidly, and Crampton’s baseline would become a waystation of inestimable value. No scientist could view such dedication in any other light. Future changes have much more value than current impressions.

One of nearly a hundred tables, most of comparable length and equally chock full of numbers, from Crampton’s monograph on the
Partula
of Moorea. Each number in the chart is a calculated average based on many specimens, not simply a measurement.
Carnegie institution of Washington
.

And Crampton’s plan paid off—or so it seemed at first. Three of the world’s finest biologists of land snails took up the study of
Partula
in the next generation, building explicitly on Crampton’s work—Bryan Clarke of the University of Nottingham, Jim Murray of the University of Virginia, and Mike Johnson of the University of Western Australia. They have published numerous papers, in varying combinations of authorship, from the mid-1960s to the present day. Working primarily on everyone’s favorite island of Moorea, they have made important revisions to Crampton’s conclusions and have added great sophistication in mathematical procedures (now computerized) and genetical methods not available to Crampton. In 1980, Murray and Clarke ended an important paper, “The genus
Partula
on Moorea: Speciation in progress,” with these words:

Add snails to Burns’ litany about the best laid plans of mice and men. Great expectations die quickly on the bonfires of human vanity. We are only a decade from these brave words of 1980, but Moorea is no longer a laboratory for studying active speciation in
Partula
. It has become a mausoleum.

Think of all the metaphors you know for little things made worse by attempted solutions that cascade to even greater problems, for you need this apparatus to grasp the extirpation of
Partula
on Moorea. Think of Pandora’s box. Think of the old woman who swallowed a fly in the folk song. (She then swallowed a spider to catch the fly, a bird to catch the spider, a cat to catch the bird…and up the size range of the animal kingdom. Each successive verse gets longer as singers run through the full range of ingestions, but the last is stunningly brief: “There was an old lady who swallowed a horse. She died, of course.”)

Partula
eats fungi growing upon dead vegetation and poses no threat whatever to agriculture. Its only, and slight, impact upon the native economy is entirely positive, as women string the shells together to make leis for the tourist trade. But animals introduced onto isolated islands often play havoc both with native organisms and with agriculture, witness the rabbits of Australia and, to cite the most dangerous creature of all, the humans who wiped out so many species of moas on New Zealand. An introduced snail began the sad chain of destruction on Moorea.

In sharp contrast with the benign
Partula
, African tree snails of the genus
Achatina
are, in almost all cases, unmitigated disasters. First of all, they are gigantic (as snails go); second, they are voracious herbivores of living plants, including many agriculturally important species. With their clear record of destruction on island after island, I am amazed that people still introduce them purposefully. (They are brought in for food, for I’m told that they are succulent, and you do get a lot of meat per creature.)
Achatina
was first imported to the Indo-Pacific realm in 1803 by the governor of Réunion who brought them from Madagascar so that a lady friend could continue to enjoy snail soup. They escaped from his garden and devastated the island. By 1847, they had reached India. In the 1930s, they began to spread into South Pacific islands, usually by purposeful introduction for food.

Achatina fulica
reached Tahiti in 1967 and soon spread to neighboring islands. By the mid-1970s, the infestation had become particularly serious on Moorea. The snails even invaded human dwellings; one report tells of a farmer who removed two wheelbarrow-loads of
Achatina
from the walls of his house. Admittedly, something had to be done. But
que faire
, as they say in this very French land?