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14

The Nature of Living Things

The word “evolution” was originally coined to describe the unfolding of form

and function from a single-celled zygote to a multicelled adult organism (“normal

development”). Since it happens daily and can be observed in the laboratory, it is far

more amenable to detailed scientific study than evolution comprising speciation and

extinction over geological timescales.

The notion of evolution as the unfolding of parts believed to be already existent

in compact form had already been formalized in 1764 by Bonnet under the name of

preformation, and had been given a rather mechanical interpretation (i.e., unfolding

of a highly compact homunculus produced the adult form).

Later, the term (evolution) came to be used to signify the epigenetic aspects of

development. Epigenesis became the alternative to preformation, with the connota-

tion of “order out of chaos”. Both preformation and epigenesis contained the notion

of coded instructions, but in the latter, at the time of its formulation the actual mech-

anism was conceived rather vaguely (e.g., by suggesting the coöperation of “inner

and outer forces”). Nevertheless, it was firmly rooted in the notion of entelechy; in

other words, the emphasis was on the potential for development, not on a determin-

istic path, which is entirely compatible with the cellular automaton interpretation of

development. One might also refer to the interaction of genes with their environ-

ment. ⁵⁹ “Environment” includes constraints set by the physical chemistry of matter

in general. Wilhelm His clearly perceived the importance of general mechanical

considerations in constraining morphology.

The term “ontogeny” was coined by Ernst Haeckel to signify the developmental

history of an individual, as opposed to “phylogeny”, signifying the evolution of a

type of animal or plant (i.e., the developmental history of an abstract, genealogical

individual).

It has been an important guiding principle that ontogeny is a synopsis of phy-

logeny. Very extensive observations of developing embryos in the eighteenth and

nineteenth centuries led to a number of important empirical generalizations, such

as von Baer’s laws of development (e.g., “special features appear after the general

ones”). It was clear that development embodied different categories of processes

with different timescales largely uncoupled from one another: simple growing (the

isometric increase of size); growing up (allometric increase, ⁶⁰ especially important in

the development of the embryo); and growing older (maturation). By adjusting these

timescales relative to each other (heterochrony), different forms could be created.

Much debate has centred around neoteny—the retention of juvenile features in

the adult animal (paedomorphosis)—and progenesis—the truncation of ontogeny by

precocious sexual maturation. They can be thought of as, respectively, retardation

and acceleration of development. If organ size (yy) is plotted against body size (xx)

59 This is a very basic notion that crops up throughout biology. At present, there is no satisfactory

universal formulation, however, but many interesting models have been proposed and investigated,

including those of Érdi and Barna (1984) for neurogenesis, and Luthi et al. (1998) for neurogenesis

in Drosophila. All of these models reduce to the basic formulation for the regulator (Sect. 3.2),

discussed by Ashby (1956).

60 Allometric relations are of the typey equals b x Superscript ay = bxa, whereaa andbb are constants.a equals 1a = 1 corresponds to

isometry.