1206 THE STRUCTURE OF EVOLUTIONARY THEORY
"conclusion" (pp. 607-611), he decries the standard tendency of the time (now
happily abandoned, but mostly for reasons other than D'Arcy Thompson's critique) to
establish speculative phylogenies not so much on stratigraphic successions, but on
idealized series moving from simple forms like the spherical Orbulina to ever more
complex designs, and with attributions of increasing adaptive value (usually for
greater test strength) to these more elaborate forms. Instead, D'Arcy Thompson views
the diversity of foram shells as so many incarnations of permitted designs under the
shaping rules that actually direct the building of tests. Since these rules do not change
through time, the potential forms remain immanent in nature, and become occupied
again and again, throughout the history of the entire lineage, by various clades that
fall under the relevant set of forces. These forms are therefore no more arrangeable
from worse to better, and no more subject to specific accounts of historical filiation,
than are the varied shapes of snowflakes or quartz crystals. (I particularly like this
"riff" because D'Arcy Thompson admits that phylogenies for complex metazoans
may be better founded, and because he may well be right that, for these simplest
creatures of the fossil record, repeated shapes in widely separated times and places
may represent massive homoplasy based on the ease of revolving certain basic
designs. After all, a hippo is only a hippo and will evolve just once, but the form of a
microscopic calcareous sphere, floating in the ocean, may be attainable along many
routes—pp. 610-611):
The theorem of Organic Evolution is one thing; the problem of deciphering
the lines of evolution, the order of phylogeny, the degrees of relationships and
consanguinity, is quite another. Among the higher organisms we arrive at
conclusions regarding these things by weighing much circumstantial evidence,
by dealing with the resultant of many variations, and by considering the
probability or improbability of many coincidences of cause and effect...
But in so far as forms can be shown to depend on the play of physical
forces, and the variations of form to be directly due to simple quantitative
variations in these, just so far are we thrown back on our guard before the
biological conception of consanguinity, and compelled to revise the vague
canons, which connect classification with phylogeny.
The physicist explains in terms of the properties of matter, and classifies
according to a mathematical analysis, all the drops and forms of drops and
associations of drops, all the kinds of froth and foam, which he may discover
among inanimate things; and his task ends there. But when such forms, such
conformations and configurations, occur among living things, then at once the
biologist introduces his concepts of heredity, of historical evolution, of
succession in time ... if fitness for a function, of adaptation to an environment,
of higher and lower, of "better" and "worse." This is the fundamental
difference between the "explanations" of the physicist and those of the
biologist.
In the order of physical and mathematical complexity there is no question
of