pulled toward the rear as the embryo’s 
top surface expanded backward while 
its belly grew little. An adult stripe, 
subject to such deformation in its em- 
bryonic life, would be broad and would 
run from the belly up and over the 
haunch — as in Burchell’s zebra. 
Thus, Bard can explain differences in 
rear striping of the three species as the 
results of deforming the same initial 
pattern at different times during nor- 
mal embryonic growth. His hypothesis 
receives striking support from another 
source: the total number of stripes it- 
self. Remember that Bard assumes a 
common size and spacing for stripes at 
the outset. Thus, the larger the embryo 
when stripes first form, the greater the 
number of stripes. Grevy’s zebra, pre- 
sumably forming its stripes as an em- 
bryo of five weeks and about 32 mm, 
has 80 or so stripes as an adult — or 
about 0.4 mm per stripe. Mountain ze- 
bras with a fourth-week embryo of 
some 14 to 19 mm in length have about 
43 stripes — again about 0.4 mm per 
stripe. Burchell’s zebra has 25 to 30 
stripes; if they form in a third-week em- 
bryo some 1 1 mm long, we get the same 
value — about 0.4 mm per stripe. 
As additional support, and a lovely 
example of the difference between su- 
perficial appearance and knowledge of 
underlying causes, consider an old par- 
adox involving hybrid offspring be- 
tween zebras and true horses. These 
animals almost always have more 
stripes than their zebra parent. “Com- 
mon sense,” based on superficial ap- 
pearance, declares this result puzzling. 
After all, the state in between stripes 
and no stripes is few stripes. But if Bard 
is right about the underlying causes of 
striping, then this paradoxical result 
makes sense. The intermediate state be- 
tween stripes and no stripes might well 
be a delay in the embryonic formation 
of stripes. If stripes then form at their 
“ Transformed 
coordinates" on 
the carapaces 
of crabs 
from two 
different 
genera 
demonstrate 
unity 
of form. 
From On Growth and Form, by D'Arcy Wentworth Thompson/Cambridge 
University Press, 1917 
common size and spacing upon a larger 
embryo, the resultant adult will have 
more stripes. 
If a unity of basic architecture under- 
lies the diversity of zebra striping, then 
we must suspect that we are confront- 
ing a general pattern in nature, not just 
the “supreme abnormality” that Mari- 
anne Moore described. Darwin viewed 
horses in this light and recognized that 
the capacity for striping in all horses 
constituted a powerful argument for 
evolution itself. (As in last month’s col- 
umn, I use horse as a general term for 
all members of the family Equidae — 
true horses, asses, and zebras. When I 
wish to indicate Old Dobbin alone, I 
will speak of true horses.) If zebras are 
odd and perfect adaptations for camou- 
flage, God might have made them as 
we find them. But if zebras merely actu- 
ate and exaggerate a potential property 
of all horses, then the occasional real- 
ization of striping in other horses — 
where it cannot be viewed as a perfected 
adaptation ordained by God — must 
indicate a community of evolutionary 
descent. 
Darwin devoted much space in chap- 
ter 5 of the Origin of Species to an ex- 
haustive tabulation of occasional 
striping in other horses. Asses, he 
found, often have “very distinct trans- 
verse bars . . . like those on the legs of a 
zebra.” True horses often possess a spi- 
nal stripe, and some also have trans- 
verse leg bars. Darwin found a Welsh 
pony with three parallel stripes on each 
shoulder. And he noted that hybrids 
(with no zebra parents) were often 
strongly striped — an example of the 
common, and still mysterious, observa- 
tion that hybrids often display ancestral 
reminiscences present in neither par- 
ent. “I once saw a mule,” Darwin 
wrote, “with its legs so much striped 
that any one at first would have 
thought that it must have been the 
product of a zebra.” 
From this illustration of common, 
and often nonadaptive, patterns in all 
horses, Darwin drew one of his most 
powerful and passionate arguments for 
evolution — well worth quoting in ex- 
tenso: 
He who believes that each equine species 
was independently created, will, I presume, 
assert that each species has been created 
with a tendency to vary, both under nature 
and under domestication, in this particular 
manner, so as often to become striped like 
other species of the genus; and that each has 
been created with a strong tendency, when 
crossed with species inhabiting distant 
quarters of the world, to produce hybrids 
resembling in their stripes, not their own 
parents, but other species of the genus. To 
admit this view is, as it seems to me, to reject 
a real for an unreal, or at least for an un- 
known, cause. It makes the works of God a 
mere mockery and deception; I would al- 
most as soon believe with the old and igno- 
rant cosmogonists, that fossil shells had 
never lived, but had been created in stone so 
as to mock the shells now living on the sea- 
shore. 
The same theme also suggests an an- 
swer to the title of last month’s essay: 
“What, if Anything, Is a Zebra?” I ad- 
vanced the argument that zebras may 
not form a group of closest relatives, 
but a set of different horses that had ei- 
ther evolved stripes independently or 
inherited them from a common ances- 
tor (while asses and true horses lost 
them). Bard’s hypothesis lends support 
to this conjecture because it suggests 
that the underlying pattern of zebra 
striping may be so simple that all horses 
include it in their repertoire of develop- 
ment. Zebras, then, may be the realiza- 
tion of a potential possessed by all 
horses. 
Finally, moving from the sublime to 
the merely interesting, Bard proposes a 
solution to the primal dilemma and ar- 
gues that zebras are black animals with 
white stripes after all. The white under- 
belly, he points out, is a lousy argument 
because many fully colored mammals 
are white underneath. Color may be 
generally inhibited in this region for 
reasons at present unknown. Mammals 
do not have their colors painted on a 
white background. The basic issue may 
then be rephrased: Does striping result 
from an inhibition or a deposition of 
melanin? If the first, zebras are black 
animals; if the second, they are white 
with black stripes. 
Biologists often look to teratologies, 
or abnormalities of development, to 
solve such issues. Bard has uncovered 
an abnormal zebra whose “stripes” 
consist of rows of dots and discontinu- 
ous blotches, rather than coherent lines 
of color. The dots and blotches are 
white on a black background. Bard 
writes: “It is only possible to under- 
stand this pattern if the white stripes 
had failed to form properly and that 
therefore the ‘default’ color is black. 
The role of the striping mechanism is 
thus to inhibit natural pigment forma- 
tion rather than to stimulate it.” The 
zebra, in other words, is a black animal 
with white stripes. 
Stephen Jay Gould teaches biology, geol- 
ogy, and the history of science at Har- 
vard University. 
22 
