much lip service in textbooks — during 
the past forty years, while evolutionary 
biologists have generally preferred a 
rather strict construction of Darwinism 
in their explanations of nature. 
For many reasons, ranging from the 
probable neutrality of much genetic 
variation to the nonadaptive nature of 
many evolutionary trends, this strict 
construction is breaking down, and 
themes of unity are receiving renewed 
attention. Old ideas are being rediscov- 
ered; D’Arcy Thompson, although 
never out of print, is now often out of 
bookstores (and in personal libraries). 
One old and promising theme empha- 
sizes the correlated effects of changes in 
the timing of events in embryonic de- 
velopment. A small change in timing, 
perhaps the result of a minor genetic 
modification, may have profound ef- 
fects on a suite of adult characters if the 
change occurs early in embryology and 
its effects accumulate thereafter. 
The theory of human neoteny, often 
discussed in these columns, is an ex- 
pression of this theme. It holds that a 
slowdown in maturation and rates of 
development has led to the expression 
in adult humans of a large suite of fea- 
tures generally found in embryos or ju- 
venile stages of other primates. Not all 
these features need be viewed as direct 
adaptations built by natural selection. 
Many, like the “embryonic” distribu- 
tion of body hair on heads, armpits, 
and pubic regions or the preservation of 
an embryonic membrane, the hymen, 
through puberty, may be nonadaptive 
consequences of a basic neoteny that is 
adaptive for other reasons — the value 
of slow maturation in a learning ani- 
mal, for example. 
Bard’s proposal for “a unity underly- 
ing the different zebra striping pat- 
terns” ( Journal of Zoology, London, 
vol. 183, 1977, pp. 527-39) follows 
D’Arcy Thompson’s theme of a basic 
pattern stretched and pulled in differ- 
ent ways by varying forces of embryon- 
ic growth. These varying forces arise 
because the basic pattern develops at 
different times in the embryology of the 
three species. Bard thus combines the 
theme of transformed coordinates with 
the insight that substantial evolution 
can proceed by changes in the timing of 
development. 
The basic pattern is simplicity itself: 
a series of parallel stripes deposited per- 
pendicular to a line running along the 
embryonic zebra’s back from head to 
tail — hang a sheet over a taut wire and 
paint vertical stripes on each side of it. 
These stripes are initially laid down at a 
constant size, no matter how big the 
embryo that forms them. They are 0.4 
mm, or approximately 20 cell diame- 
ters, apart. The bigger the embryo, the 
greater the initial number of stripes. (I 
should point out that Bard’s argument 
is a provocative model for testing, not a 
set of observations; no one has ever 
traced the embryology of zebra striping 
directly.) 
The three zebra species differ in both 
number and configuration of stripes. In 
Bard’s hypothesis, these complex vari- 
ations arise only because the same basic 
pattern — the parallel stripes of con- 
stant spacing — develops during the 
fifth week of embryonic growth in one 
species, during the fourth week in an- 
other, and during the third week in the 
third species. Since the embryo under- 
goes complex changes in form during 
these weeks, the basic pattern is 
stretched and distorted in varying ways 
in the three species, leading to all the 
major differences in adult striping. 
The three species differ most notably 
in patterns of striping on the rump and 
hind quarters. The Grevy’s zebra 
{Equus grevyi ) has numerous fine and 
basically parallel stripes in these rear 
regions. On Bard’s model, the stripes 
must have formed when the back part 
of the embryo was relatively large. (The 
larger the part, the more stripes it re- 
ceives, since stripes are initially formed 
at constant size and spacing.) In the 
embryology of horses, the tail and hind 
regions expand markedly during the 
fifth week in utero. If adults possess nu- 
merous, fine posterior stripes, they 
must form after this embryonic expan- 
sion of the rear quarters. (Unfortunate- 
ly, no one has ever studied the early 
embryology of zebras directly, and 
Bard assumes that the intrauterine 
growth patterns of true horses are fol- 
lowed by their striped relatives as well. 
Since basic features of early embryolo- 
gy tend to be so conservative in evolu- 
tion, true horses are probably fair 
models for zebras.) 
The mountain zebra, Equus zebra, 
looks much like E. grevyi until we reach 
the haunch, where three broad stripes 
substitute for the numerous fine stripes 
of Grevy’s zebra. Broad stripes on 
adults indicate initial formation on a 
small piece of embryo (where few 
stripes could fit), and later rapid 
growth of the piece (widening the 
stripes as the general area expands). If 
an embryo forms stripes in its fourth 
week, just before the posterior expan- 
sion that provides room for the many 
fine marks of Grevy’s zebra, it will 
build the pattern of a mountain zebra 
during later embryonic growth. 
Burchell’s zebra, Equus burchelli, 
also has just a few broad stripes on its 
haunch. But, while the mountain zebra 
has fine stripes over most of its back 
and broad stripes only over the haunch, 
the broad stripes of Burchell's zebra be- 
gin in the middle of the belly and sweep 
back over the haunch. This pattern sug- 
gests an initial formation of stripes 
during the third week of embryonic 
growth. At this early stage, the embryo 
has a short, compact back, which later 
expands toward the rear in a broad, 
arching curve while the belly remains 
short. A stripe that initially ran verti- 
cally from spine to belly would be 
17 
