Leonardo’s Mountain of Clams and the Diet of Worms (39 page)
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Authors: Stephen Jay Gould
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Darwin provides a cogent and subtle explanation (perhaps not thoroughly satisfactory by current
views, but entirely logical and coherent). Interestingly, his proposal embodies the theme of this essay—the need to dissociate current utility from historical origin. A giraffe’s current functioning may require coordinated action of all parts that support the long neck, but these features need not have evolved in lockstep. If the neck grows by ten feet all at once, then every supporting bit of anatomy
must be in place. But if the neck elongates an inch at a time, then the full panoply of supporting structures need not arise at every step. The coordinated adaptation can be built piecemeal. Some animals may slightly elongate the neck, others the legs; still others may develop stronger neck muscles. By sexual reproduction, the favorable features of different organisms may be combined in offspring.
views, but entirely logical and coherent). Interestingly, his proposal embodies the theme of this essay—the need to dissociate current utility from historical origin. A giraffe’s current functioning may require coordinated action of all parts that support the long neck, but these features need not have evolved in lockstep. If the neck grows by ten feet all at once, then every supporting bit of anatomy
must be in place. But if the neck elongates an inch at a time, then the full panoply of supporting structures need not arise at every step. The coordinated adaptation can be built piecemeal. Some animals may slightly elongate the neck, others the legs; still others may develop stronger neck muscles. By sexual reproduction, the favorable features of different organisms may be combined in offspring.
In developing this general explanation by using the giraffe as a putative example, Darwin does engage in conjectural biology. But I would defend this mode of speculation as a device utterly different from telling fatuous stories. When scientists need to explain difficult points of theory, illustration by hypothetical example—rather than by total abstraction—works well (perhaps indispensably) as
a rhetorical device. Such cases do not function as “speculations” in the pejorative sense—as silly stories that provide no insight into complex mechanisms—but rather as idealized illustrations to exemplify a difficult point of theory. (Other fields, like philosophy and the law, use such conjectural cases as a standard device.)
a rhetorical device. Such cases do not function as “speculations” in the pejorative sense—as silly stories that provide no insight into complex mechanisms—but rather as idealized illustrations to exemplify a difficult point of theory. (Other fields, like philosophy and the law, use such conjectural cases as a standard device.)
In thus invoking the giraffe as a proper exemplification, Darwin does
embed a line within his text about adaptive advantages of reaching high. Taken out of context, this comment could be read as a premonition of silly speculations to come. But its role as part of a conjectural case to illustrate a more subtle point of theory should be clear in the following totality (from Darwin’s 1868 book, volume 2, pages 220-221):
embed a line within his text about adaptive advantages of reaching high. Taken out of context, this comment could be read as a premonition of silly speculations to come. But its role as part of a conjectural case to illustrate a more subtle point of theory should be clear in the following totality (from Darwin’s 1868 book, volume 2, pages 220-221):
With animals such as the giraffe, of which
the whole structure is admirably coordinated for certain purposes, it has been supposed that all the parts must have been simultaneously modified; and it has been argued that, on the principle of natural selection, this is scarcely possible. But in thus arguing, it has been tacitly assumed that the variations must have been abrupt and great. No doubt, if the neck of a ruminant were suddenly to become
greatly elongated, the fore-limbs and back would have to be simultaneously strengthened and modified; but it cannot be denied that an animal might have its neck, or head, or tongue, or fore-limbs elongated a very little without any corresponding modification in other parts of the body; and animals thus slightly modified would, during a dearth, have a slight advantage, and be enabled to browse
on higher twigs, and thus survive. A few mouthfuls more or less every day would make all the difference between life and death. By the repetition of the same process, and by the occasional intercrossing of the survivors, there would be some progress, slow and fluctuating though it would be, towards the admirably coordinated structure of the giraffe.
the whole structure is admirably coordinated for certain purposes, it has been supposed that all the parts must have been simultaneously modified; and it has been argued that, on the principle of natural selection, this is scarcely possible. But in thus arguing, it has been tacitly assumed that the variations must have been abrupt and great. No doubt, if the neck of a ruminant were suddenly to become
greatly elongated, the fore-limbs and back would have to be simultaneously strengthened and modified; but it cannot be denied that an animal might have its neck, or head, or tongue, or fore-limbs elongated a very little without any corresponding modification in other parts of the body; and animals thus slightly modified would, during a dearth, have a slight advantage, and be enabled to browse
on higher twigs, and thus survive. A few mouthfuls more or less every day would make all the difference between life and death. By the repetition of the same process, and by the occasional intercrossing of the survivors, there would be some progress, slow and fluctuating though it would be, towards the admirably coordinated structure of the giraffe.
I suspect that the giraffe’s neck first became
an explicit and contested issue within evolutionary theory when St. George Mivart, a fascinating rebel in many ways—as a devout Catholic in this Anglican land, and as an evolutionist firmly opposed to the mechanism of natural selection—published his 1871 critique of Darwinism,
The Genesis of Species.
Mivart did focus on the giraffe’s neck, and he did present Darwin’s supposed case in the form
that has become canonical in modern high school textbooks—that is, as a speculative tale about natural selection. But note that Mivart wrote
to oppose
Darwinism, and that enemies tend to caricature and trivialize the doctrines they attack. Mivart stated:
an explicit and contested issue within evolutionary theory when St. George Mivart, a fascinating rebel in many ways—as a devout Catholic in this Anglican land, and as an evolutionist firmly opposed to the mechanism of natural selection—published his 1871 critique of Darwinism,
The Genesis of Species.
Mivart did focus on the giraffe’s neck, and he did present Darwin’s supposed case in the form
that has become canonical in modern high school textbooks—that is, as a speculative tale about natural selection. But note that Mivart wrote
to oppose
Darwinism, and that enemies tend to caricature and trivialize the doctrines they attack. Mivart stated:
At first sight it would seem as though a better example in support of “Natural Selection” could hardly have been chosen. Let the fact of the
occurrence of occasional, severe droughts in the country which that animal has inhabited be granted. In that case, when the ground vegetation has been consumed, and the trees alone remain, it is plain that at such times only those individuals (of what we assume to be the nascent giraffe species) which were able to reach high up would be preserved, and would become the parents of the following generation.
occurrence of occasional, severe droughts in the country which that animal has inhabited be granted. In that case, when the ground vegetation has been consumed, and the trees alone remain, it is plain that at such times only those individuals (of what we assume to be the nascent giraffe species) which were able to reach high up would be preserved, and would become the parents of the following generation.
For the sixth and last edition of the
Origin of Species
(1872), Darwin added the only chapter ever appended to his book—primarily to refute Mivart’s attack. This new chapter does discuss giraffes extensively (though only to rebut Mivart), and may be a primary source for the legend as later developed (for almost all reprintings and subsequent versions, up to our own day, feature this sixth edition,
and not the first edition of 1859, which did not mention giraffe necks in the context of natural selection at all).
Origin of Species
(1872), Darwin added the only chapter ever appended to his book—primarily to refute Mivart’s attack. This new chapter does discuss giraffes extensively (though only to rebut Mivart), and may be a primary source for the legend as later developed (for almost all reprintings and subsequent versions, up to our own day, feature this sixth edition,
and not the first edition of 1859, which did not mention giraffe necks in the context of natural selection at all).
When we read Darwin’s careful words, however, we encounter yet another irony in our expanding list. The giraffe’s neck supposedly supplies a crucial example for preferring natural selection over Lamarckism as a cause of evolution. But Darwin himself (however wrongly by later judgment)
did not deny the Lamarckian principle of inheritance for characters acquired by use or lost by disuse. He regarded the Lamarckian mechanism as weak, infrequent, and entirely subsidiary to natural selection, but he accepted the validity of evolution by use and disuse. Darwin does speculate about the adaptive advantage of giraffes’ necks, but he cites
both
natural selection
and
Lamarckism as probable
causes of elongation. Thus, obviously, Darwin never regarded giraffe necks as an illustration for the superiority of natural selection over other valid mechanisms. He writes in two passages of the 1872 edition, marrying Lamarck with natural selection:
did not deny the Lamarckian principle of inheritance for characters acquired by use or lost by disuse. He regarded the Lamarckian mechanism as weak, infrequent, and entirely subsidiary to natural selection, but he accepted the validity of evolution by use and disuse. Darwin does speculate about the adaptive advantage of giraffes’ necks, but he cites
both
natural selection
and
Lamarckism as probable
causes of elongation. Thus, obviously, Darwin never regarded giraffe necks as an illustration for the superiority of natural selection over other valid mechanisms. He writes in two passages of the 1872 edition, marrying Lamarck with natural selection:
By this process [natural selection] long continued . . . combined no doubt in a most important manner with the inherited effects of the increased
use of parts, it seems to me almost certain that an ordinary hoofed quadruped might be converted into a giraffe.
use of parts, it seems to me almost certain that an ordinary hoofed quadruped might be converted into a giraffe.
In every district some one kind of animal will almost certainly be able to browse higher than the others; and it is almost equally certain that this one kind alone could have its neck elongated for this purpose, through natural selection and the effects of increased use.
We may
summarize the main line of this complexly meandering tale as a list of ironies—invoking the technical definition of irony as a statement where, for humorous or sarcastic effect, the intended meaning of a word becomes directly opposite to the usual sense—as in “that’s very smart!” for a proposal you regard as consummately dumb. In this story, none of the five historical facts arose by ironic intent.
The irony occurs retrospectively, for each fact subverts the legend that “everyone knows” about tall giraffes—namely, that long necks for high leaves provide a splendid illustration for the superiority of Darwinian natural selection over Lamarckian use and disuse. The joke, in other words, is on the silly canonical legend as recounted in all modern textbooks.
summarize the main line of this complexly meandering tale as a list of ironies—invoking the technical definition of irony as a statement where, for humorous or sarcastic effect, the intended meaning of a word becomes directly opposite to the usual sense—as in “that’s very smart!” for a proposal you regard as consummately dumb. In this story, none of the five historical facts arose by ironic intent.
The irony occurs retrospectively, for each fact subverts the legend that “everyone knows” about tall giraffes—namely, that long necks for high leaves provide a splendid illustration for the superiority of Darwinian natural selection over Lamarckian use and disuse. The joke, in other words, is on the silly canonical legend as recounted in all modern textbooks.
1. Lamarck mentions giraffe necks
in one passing paragraph of speculation within a chapter devoted to much longer examples regarded as far more important.
in one passing paragraph of speculation within a chapter devoted to much longer examples regarded as far more important.
2. Darwin does not cite the case at all in the first edition of the
Origin of Species.
He does tell a giraffe story in the “just-so” mode, but from the opposite end—the tail rather than the neck. Darwin’s only quick phrase about giraffe necks illustrates the contrary theme
of inherited stability (retained number of neck vertebrae) rather than novel adaptation.
Origin of Species.
He does tell a giraffe story in the “just-so” mode, but from the opposite end—the tail rather than the neck. Darwin’s only quick phrase about giraffe necks illustrates the contrary theme
of inherited stability (retained number of neck vertebrae) rather than novel adaptation.
3. When Darwin, in his longer and more technical book of 1868, does discuss giraffe necks in the context of natural selection, he does not present the standard “just-so story” of pure speculation, but rather uses giraffes to exemplify the difficult and crucial issue of how gradualistic natural selection can
build a complex adaptation of many coordinated parts (the neck
and
all the supporting structures).
build a complex adaptation of many coordinated parts (the neck
and
all the supporting structures).
4. Mivart, in attempting to refute Darwinism, tells the “just-so story” that would become traditional, but only to caricature a theory he opposes.
5. When Darwin responds to Mivart in the last edition of the
Origin of Species
, he does interpret giraffe necks as adaptations for feeding on high leaves,
but he argues that natural selection worked in concert with Lamarckian forces! (So much for a “classic” illustration of why the giraffe’s neck leads us to prefer Darwin over Lamarck.)
Origin of Species
, he does interpret giraffe necks as adaptations for feeding on high leaves,
but he argues that natural selection worked in concert with Lamarckian forces! (So much for a “classic” illustration of why the giraffe’s neck leads us to prefer Darwin over Lamarck.)
I don’t know (but would love to find out) how and where the legend’s modern form originated in such striking contrast to alleged historical sources. Henry Fairfield Osborn, the dominant paleontologist of his era,
and longtime director of the American Museum of Natural History, gave the “standard” version in his popular book of 1918,
The Origin and Evolution of Life:
and longtime director of the American Museum of Natural History, gave the “standard” version in his popular book of 1918,
The Origin and Evolution of Life:
The cause of different bodily proportions, such as the very long neck of the tree-top browsing giraffe, is one of the classic problems of adaptation. In the early part of the nineteenth century Lamarck attributed the lengthening of the neck
to the inheritance of bodily modifications caused by the neck-stretching habit. Darwin attributed the lengthening of the neck to the constant selection of individuals and races which were born with the longest necks. Darwin was probably right.
to the inheritance of bodily modifications caused by the neck-stretching habit. Darwin attributed the lengthening of the neck to the constant selection of individuals and races which were born with the longest necks. Darwin was probably right.
This version has held ever since. Readers may well ask why we should devote energy to tracing such historical arcana. Why not let sleeping dogs lie and
silly legends propagate, especially if tall tales do no harm? I gave some theoretical reasons for interest earlier in this essay, but I also wish to stress a practical concern. If we choose a weak and foolish speculation as a primary textbook illustration (falsely assuming that the tale possesses a weight of history and a sanction in evidence), then we are in for trouble—as critics properly nail
the particular weakness, and then assume that the whole theory must be in danger if supporters choose such a fatuous case as a primary illustration. For example, in his anti-Darwinian book cited earlier (and eponymously titled
The Neck of the Giraffe
), Francis Hitching tells the story in the usual form:
silly legends propagate, especially if tall tales do no harm? I gave some theoretical reasons for interest earlier in this essay, but I also wish to stress a practical concern. If we choose a weak and foolish speculation as a primary textbook illustration (falsely assuming that the tale possesses a weight of history and a sanction in evidence), then we are in for trouble—as critics properly nail
the particular weakness, and then assume that the whole theory must be in danger if supporters choose such a fatuous case as a primary illustration. For example, in his anti-Darwinian book cited earlier (and eponymously titled
The Neck of the Giraffe
), Francis Hitching tells the story in the usual form:
The evolution of the giraffe, the tallest living animal, is often taken as classic evidence
that Darwin was right and Lamarck wrong. The giraffe evolved its long neck, it is said, because natural selection choose those animals best able to feed off the highest treetops, where there is most food and least competition.
that Darwin was right and Lamarck wrong. The giraffe evolved its long neck, it is said, because natural selection choose those animals best able to feed off the highest treetops, where there is most food and least competition.
Hitching then adds: “The need to survive by reaching ever higher for food is, like so many Darwinian explanations of its kind, little more than a
post hoc
speculation.”
Hitching is quite correct, but he rebuts a fairy story that Darwin was far too smart to tell—even though the tale later entered our high school texts as a “classic case” nonetheless. Eternal vigilance, as they say, is the price of freedom. Add intellectual integrity to the cost basis.
post hoc
speculation.”
Hitching is quite correct, but he rebuts a fairy story that Darwin was far too smart to tell—even though the tale later entered our high school texts as a “classic case” nonetheless. Eternal vigilance, as they say, is the price of freedom. Add intellectual integrity to the cost basis.
As a closing point, we might excuse this thoughtless repetition of an old legend without presumed historical
sanction, if later research had established the truth of the tale nonetheless. But when we turn to giraffes themselves, we encounter the final irony of this long story. Giraffes provide no established evidence whatsoever for how their undeniably useful necks evolved.
sanction, if later research had established the truth of the tale nonetheless. But when we turn to giraffes themselves, we encounter the final irony of this long story. Giraffes provide no established evidence whatsoever for how their undeniably useful necks evolved.
All giraffes belong to a single species, quite separate from any other ruminant mammal, and closely related only to the okapi (a
rare, short-necked, forest-dwelling species of central Africa). Giraffes have a sparse fossil record in Europe and Asia, but ancestral species are relatively short-necked, and the spotty evidence provides no insight into how the long-necked modern species arose. (
The Giraffe: Its Biology, Behavior and Ecology
, by A. I. Dagg and J. B. Foster, gives an excellent and thorough account of all major
aspects of giraffe biology.)
rare, short-necked, forest-dwelling species of central Africa). Giraffes have a sparse fossil record in Europe and Asia, but ancestral species are relatively short-necked, and the spotty evidence provides no insight into how the long-necked modern species arose. (
The Giraffe: Its Biology, Behavior and Ecology
, by A. I. Dagg and J. B. Foster, gives an excellent and thorough account of all major
aspects of giraffe biology.)
When we study the function of long necks in modern giraffes, we encounter an
embarras de richesses.
Almost anything important in the life of a giraffe involves some use of the remarkable neck. Giraffes surely employ their long necks (and their long legs, long faces, and long tongues) to reach high-growing acacia leaves. Giraffes thereby browse several feet of vegetation
exploitable by no other ground-dwelling mammal. The champion giraffe reached an astonishing nineteen feet, three inches in height. Groves of African acacia trees (I have seen this phenomenon in the field) are often denuded below a sharp line representing the highest reach of local giraffes.
embarras de richesses.
Almost anything important in the life of a giraffe involves some use of the remarkable neck. Giraffes surely employ their long necks (and their long legs, long faces, and long tongues) to reach high-growing acacia leaves. Giraffes thereby browse several feet of vegetation
exploitable by no other ground-dwelling mammal. The champion giraffe reached an astonishing nineteen feet, three inches in height. Groves of African acacia trees (I have seen this phenomenon in the field) are often denuded below a sharp line representing the highest reach of local giraffes.
But giraffes also use their necks for other prominent and crucial activities. Male giraffes, for example,
establish dominance hierarchies by frequent and prolonged bouts of “necking,” or swinging their large neck into the body of an opponent. These contests are more than merely symbolic, as the long neck propels the head with substantial force, and the bony horns atop the head can inflict considerable damage upon contact. Dagg and Foster describe a bout between two males named Star and Cream:
establish dominance hierarchies by frequent and prolonged bouts of “necking,” or swinging their large neck into the body of an opponent. These contests are more than merely symbolic, as the long neck propels the head with substantial force, and the bony horns atop the head can inflict considerable damage upon contact. Dagg and Foster describe a bout between two males named Star and Cream:
The
two bulls . . . stood side to side, head to tail, dose together, each with his legs apart under him for balance. Suddenly Star lowered his head and whipped it, horns foremost, at Cream’s trunk, connecting with an impact that was heard easily from forty meters away. Cream lurched sideways, collected himself and returned the blow with his head, striking Star on the neck. Star then aimed at Cream’s
front legs and knocked them out from under him with a blow of his head.
two bulls . . . stood side to side, head to tail, dose together, each with his legs apart under him for balance. Suddenly Star lowered his head and whipped it, horns foremost, at Cream’s trunk, connecting with an impact that was heard easily from forty meters away. Cream lurched sideways, collected himself and returned the blow with his head, striking Star on the neck. Star then aimed at Cream’s
front legs and knocked them out from under him with a blow of his head.
Dagg and Foster then describe the serious finality of potential outcomes:
The losing giraffe in such a struggle does not always escape so easily. His head may be gashed during a fight or he may be knocked to the ground unconscious . . . In such a contest in the Kruger National Park one of the contestants was killed. He
had a large hole immediately behind one ear where his top neck vertebra had been splintered by a blow; part of the splinter had pierced the spinal cord.
had a large hole immediately behind one ear where his top neck vertebra had been splintered by a blow; part of the splinter had pierced the spinal cord.
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