and usual, conclusion would view 
ribby shells on all islands as closely 
related and smooth shells as members 
of another coherent group. But we be- 
lieve that the complex set of char- 
acters forming the ribby and smooth 
morphologies arise independently, 
again and again. On the islands of 
Little Bahama Bank, both ribby and 
smooth animals share a distinctive 
genital anatomy. On the islands of 
Great Bahama Bank, both ribby and 
smooth develop an equally distinctive, 
but different, kind of penis. The ecol- 
ogy of rocky versus calm coasts may 
select for ribby and smooth morphol- 
ogies as adaptations, but the coordi- 
nated appearance of the half dozen 
distinctive traits of each morphology, 
again and again, may represent a chan- 
neling of available variation to pro- 
duce homologous series (ribby and 
smooth varieties) in different lineages 
(defined by genital anatomy). 
A complete theory of evolution must 
acknowledge a balance between “ex- 
ternal” forces of environment impos- 
ing selection for local adaptation and 
“internal” forces representing con- 
straints of inheritance and develop- 
ment. Vavilov placed too much em- 
phasis on internal constraints and 
downgraded the power of selection. 
But Western Darwinians have erred 
equally in practically ignoring (while 
acknowledging in theory) the limits 
placed upon selection by structure and 
development — what Vavilov and the 
older biologists would have called 
“laws of form.” We need, in short, 
a real dialectic between the external 
and internal factors of evolution. 
Vavilov’s personal tragedy cannot 
be undone. But he has been rehabili- 
tated in Russia, where the All-Union 
Society of Geneticists and Selection- 
ists now bears his name. We who view 
him as a martyr and champion his 
name while ignoring his ideas would 
do well to reconsider the older non- 
Darwinian tradition that he repre- 
sented. Combined with our legitimate 
conviction about the power of selec- 
tion, the principle of homologous se- 
ries (and other “laws of form”) might 
foster an evolutionary theory truly syn- 
thetic in its integration of development 
and organic form with a body of prin- 
ciples now dominated by ecology and 
the effects of selection upon single 
genes and traits. 
Stephen Jay Gould teaches biology, 
geology, and the history of science 
at Harvard University. 
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