Structural Constraints, Spandrels, and Exaptation 1201
a functionalist mechanism like natural selection (see my previous discussion of
D'Arcy Thompson's contemporary, William Bateson, pp. 396-415).
On the theme of constraint, D'Arcy Thompson frequently argues that once one
accepts a more than analogical comparison between the good designs, based on
idealized geometry, of organic and inorganic objects, one must also defend (not
apologetically, but for the utility of the conclusion in understanding anatomical order
and taxonomic interrelationships) the limitation of organic morphology to channels of
transformation that the physical causes of morphogenesis must follow (p. 137): "The
world of things living, like the world of things inanimate, grows of itself, and pursues
its ceaseless course of creative evolution. It has room, wide but not unbounded, for
variety of living form and structure, as these tend towards their seemingly endless,
but yet strictly limited, possibilities of permutation and degree."
Once these directing channels of evolution have been established by physical
forces governing the changes, we have no reason to restrict our concept of movement
down these channels to the slow and continuous pacing of Darwinian gradualism.
Relevant physical laws will regulate the path, "be the journey taken fast or slow." In
the following passage (p. 155), D'Arcy Thompson begins to link his model of
channeled discontinuity to critiques of specific principles within evolutionary
theory—in this case to the prevalent idea that similarities between ontogeny and
phylogeny must arise for phyletic or historical reasons. Why introduce this "extra"
and superfluous hypothesis, D'Arcy Thompson asks, if the same physical law, "caring
little" whether it works during the growth of individuals or the evolution of lineages,
mandates the same changes in any separate case:
The differences of form, and changes of form, which are brought about by
varying rates (or "laws") of growth, are essentially the same phenomenon
whether they be, so to speak, episodes in the life-history of the individual, or
manifest themselves as the normal and distinctive characteristics of what we
call separate species of the race. From one form, or ratio of magnitude, to
another there is but one straight and direct road of transformation, be the
journey taken fast or slow; and if the transformation take place at all, it will in
all likelihood proceed in the self-same way, whether it occur within the life-
time of an individual or during the long ancestral history of a race.But for sheer iconoclasm, nothing in all of D'Arcy Thompson's prose matches
the section that he added as "conclusion" to the chapter on transformed coordinates in
the second (1942) edition of Growth and Form—a "great quotation" well worth citing
in extenso. Here he makes his most incisive and forthright linkage between the two
great structuralist themes of channeling and saltational discontinuity. Taxonomic
groups of organisms may be compared (truly "homologized" in the conceptual sense
of being regulated by the same causal principle) with families of mathematical curves
generated by definite parameters of construction. Complete continuity may reign
within a family, but the gaps between can only be crossed per saltum. D'Arcy