could predict the existence of undis- 
covered varieties within one species 
after finding their parallel forms in 
another species. In 1916, for example, 
he found several varieties of wheat 
without ligules in Afghanistan (ligules 
are thin membranes that grow from 
the base of the leaf blade and surround 
the stem in many grasses). This dis- 
covery suggested that varieties of rye 
without ligules should also exist, and 
he grew them from seeds collected 
in Pamir in 1918. He predicted that 
durum wheat (Triticum durum), then 
represented exclusively by spring va- 
rieties, should also have winter forms 
since related species do — and he found 
them in 1918 in an isolated region 
of northern Iran. 
Vavilov’s observation would have 
engendered less controversy had he 
not interpreted it, indeed overinter- 
preted it, in a manner uncongenial 
with strictly Darwinian or Lamarckian 
views. He might have argued that his 
series of parallel varieties represented 
similar adaptations of different ge- 
netic systems to common environ- 
ments that engendered natural selec- 
tion in the same direction. Such an 
interpretation would have satisfied 
Darwinian preferences for random 
variation, with evolutionary change 
imposed by natural selection. (It could 
also have been distorted by Lysenko 
into a claim that environments directly 
altered the heredity of plants in fa- 
vorable ways.) 
But Vavilov proposed a different 
explanation more in tune with non- 
Darwinian (though not anti-Darwin- 
ian) themes still popular during the 
1920s: he claimed that the parallel 
series of varieties represented identical 
responses of the same genetic systems, 
inherited in toto from species to re- 
lated species. Thus, in evolutionary 
parlance, his series were “homol- 
ogous” — hence the name of his law. 
(Homologies are similarities based on 
inheritance of the same genes or struc- 
tures from a common ancestor. Simi- 
larities forged within different genetic 
systems by selective pressures of simi- 
lar environments are called analogies.) 
Vavilov argued that new species 
arise by developing genetic differences 
that preclude interbreeding with re- 
lated species. But the new species is 
not genetically distinct from its an- 
cestor in all ways. Most of the an- 
cestor’s genetic system remains intact; 
only a limited number of genes are 
altered. The parallel varieties, then, 
represent a “playing out” of the same 
genetic capacities inherited as blocks 
from species to related species. 
Such an interpretation is not anti- 
Darwinian because it does not deny 
an important role to natural selection. 
While each variety may represent a 
predictable latent capacity, its expres- 
sion in any climate or geographical 
region still requires selection to pre- 
serve the adaptive variant and to elimi- 
nate others. But such an explanation 
does conflict with the spirit of strict 
Darwinism because it weakens or com- 
promises the cardinal tenet that se- 
lection is the creative force in evo- 
lution. Random, or undirected, var- 
iation plays a crucial role in the Dar- 
winian system because it establishes 
the centrality of selection by guar- 
anteeing that evolutionary change can- 
not be ascribed to variation itself. 
Variation is only raw material. It arises 
in all directions or, at least, is not 
preferentially ordered in adaptive 
ways. Hence, direction is imposed by 
natural selection, slowly preserving 
and accumulating, generation after 
generation, the variations that render 
organisms better adapted to local en- 
vironments. 
But what if variation is not fortu- 
itous and undirected but strongly 
channeled along certain paths. Then 
only a limited number of changes are 
possible, and they record the “inter- 
nal” constraints of inheritance as 
much as the action of selection. Se- 
lection is not dormant; it still deter- 
mines which of several possibilities 
reaches expression in any one climate 
or area. But if possibilities are strictly 
limited, and if a species displays all 
of them among its several varieties, 
then this range of form cannot be as- 
cribed only to selection acting upon 
fortuitous variation. 
Moreover, this explanation for new 
varieties undermines the cardinal prin- 
ciple of creativity for natural selection. 
The variations are predictable results 
within their genetic system. Their oc- 
currence is almost foreordained. The 
role of natural selection is negative. 
It is an executioner only. It eliminates 
the variants unfit in any given en- 
vironment, thus preserving the favored 
form that had to arise eventually. 
Vavilov interpreted his law of ho- 
mologous series in this non-Darwinian 
manner. “Variation,” he wrote, “does 
not take place in all directions, by 
chance and without order, but in dis- 
tinct systems and classes analogous 
to those of crystallography and chem- 
istry. The same great divisions [of or- 
ganisms] into orders and classes mani- 
fest regularities and repetitions of sys- 
tems.” He cites the case of “several 
varieties of vetches so similar to or- 
dinary lentils in the shape, color, and 
size of their seeds, that they cannot 
be separated by any sorting machine.” 
He agrees that the extreme similarity 
in any one spot is a product of se- 
lection — unconscious selection in ag- 
ricultural sorting machines. But the 
agent of selection was, literally in this 
case, only a sieve that preserved one 
variant among many. The proper vari- 
ant existed already as a realized prod- 
uct of an inherited set of possibilities. 
The role of natural selection in this case 
is quite clear. Man unconsciously, year 
after year, by his sorting machines sep- 
arated varieties of vetches similar to len- 
tils in size and form of seeds, and ripening 
simultaneously with lentils. The same va- 
rieties certainly existed long before se- 
lection itself, and the appearance of their 
series, irrespective of any selection, was 
in accordance with the laws of variation. 
Vavilov, overenthused with his own 
idea, went on. He became intoxicated 
with the notion that his law might 
represent a principle of ordering that 
would render biology as exact and ex- 
perimental as the “hard” sciences of 
physics and chemistry. Perhaps ge- 
netic systems are composed of “ele- 
ments.” Perhaps the geographical va- 
rieties of species are predictable “com- 
pounds” that arise inevitably from the 
union of these elements in specified 
mixtures. If so, the ranges of biological 
form within a species might be laid 
out in a table of possibilities similar 
to the periodic table of chemical ele- 
ments. Evolution might be deduced 
from genetic structure itself; environ- 
ment can only act to preserve inherent 
possibilities. 
He advocated an explicit “analogy 
with chemistry” in the concluding sec- 
tion of his 1922 paper and wrote: 
“New forms have to fill vacancies in 
a system.” He experimented with a 
style of notation that expressed va- 
rieties of a species as a chemical for- 
mula and advocated “the analogy of 
homological series of plants and ani- 
mals with systems and classes of crys- 
tallography with definite chemical 
structures.” One zealous supporter 
commented that “biology has found 
its Mendeleev.” 
Vavilov moderated his views during 
the 1920s and early 1930s. He learned 
that some of the parallel varieties be- 
tween species are not based upon the 
same genes after all, but represent 
16 
