374 Michael Wheeler
Having said that, not all modes of causal spread are quite so obviously harmless
to representational explanation. Consider what one might callnon-trivial causal
spread. This phenomenon arises when the newly discovered additional causal fac-
tors reveal themselves to be at the root of some distinctive target feature of the
phenomenon of interest. In effect, where one confronts non-trivial causal spread,
a new sharing-out of the explanatory weight is mandated. Call thisexplanatory
spread[Wheeler and Clark, 1999]. Mameli [2005] explains the key points in this
way.
Causal spread occurs when we discover some new factor causally in-
volved in the occurrence of a phenomenon. Explanatory spread occurs
when we realize that some factor that was not considered to be nec-
essary in the explanation of a phenomenon is instead explanatorily
necessary for that phenomenon. Or, to put it differently, explanatory
spread occurs when we realize that some factor that was not taken to
be part of a sufficient explanation of a phenomenon needs to be in-
cluded in such explanation. Since the fact that something is causally
required does not entail that it is also explanatorily required, causal
spread does not necessarily lead to explanatory spread. But in cases
where the newly discovered causal factor is deemed to be an important
one, causal spread is likely to generate the inclusion of the newly dis-
covered factor in any sufficient explanation of a phenomenon to which
this factor causally contributes. That is, in these cases, causal spread
leads to explanatory spread. [Mameli, 2005, 388]In the present paper, the phenomenon of interest is organismic structure, and
the default position is that such structure is down to genetic coding (on some-
thing like a Lorenzian model according to which the non-genetic material causes
in development are the bricks and mortar out of which the organism is assembled
according to the genetic blueprint). Against this background, one would have non-
trivial causal spread where one discovered a distributed developmental system in
which non-genetic organismic and/or wider environmental factors made explanato-
rily non-negligible contributions to phenotypic form. So, is there non-trivial causal
spread, and thus explanatory spread, in (our theories of) biological development?
The answer, surely, is yes. Developmental explanatory spread is common. I shall
give just a few brief illustrative examples, but the biological literature is simply
brimming over with others.
First, consider the process of determination during cell specialization. In ver-
tebrates, prior to the third cleavage stage, the cells in the developing embryo
retain the possibility of achieving any of the full range of developmental outcomes
available to the original zygote. The process of determination, in which the fu-
ture course of development in the cells is differentially restricted, depends on a
process in which the nuclei of the various cells become embedded in different cyto-
plasmic environments which in turn have different regulatory effects on the genes
within the various nuclei. The sources of this differential embedding are a range