Heterochrony and Evolutionary Processes

Historical Antecedents

A Tale of Two Metaphysics




Historical Antecedents

Recognising Heterochrony

Modern examples: Sexual Dimorphism

Cambrian trilobites

Cope's Rule

K- and r- selection: Tertiary echinoids

Consequences for debates on adaptation, constraints and evolutionary dynamics


The Biogenetic Law may be thought of as encompassing two perennial themes in Western thought, particularly with respect to biology. The first is the notion of a "Great Chain of Being", where the natural order is understood in terms of a hierarchy. All species occupy a specific place in an ascending order from most simple and imperfect to most complex and perfect. Plants are "lower" than animals for the latter are mobile. Humans are the most perfect of animals, for they have capacity to understand. Typically, this scheme also allows for positing different levels of human advancement, the specific order of which tended to reflect whatever racist and/or biases were operative at the time (Aristotle held Greeks to be far superior to barbarians, for the former could form meaningful political associations and the latter were only fit for slavery; Haeckel and the Naturaphilosophie crowd had Germanic Europeans at the top).

This picture, dating from the nineteenth century, shows a typical "progression" of life-forms. Note how a snipe is placed in the same lineage as a crocodile, several dog breeds and then primates (!). The upper column headings are obscured. They read: neanderthal, negro, american savage, asiatic and then several european races, in the typical racist fashion of the time (from Gould (1987)).

Plato and Aristotle.

(Two very snappy dressers and excellent conversationalists to boot)

The second metaphysic that informs the Biogenetic Law is termed by Gould (1977) as the "Analogistic Tradition". The idea here is that the microcosm reflects the macrocosm: the motions of heavenly bodies follow the same principles as guide biological interactions, which is itself mirrored in development and function at the level of the organism. If the Great Chain of Being is an Aristotelian idea, the notion that the order inherent in the world appears at every level of organization or description is Platonic.

The Biogenetic Law falls out of these two ideas. If you believe that the world is animated by a perfecting and organizing Geist - that is, that creation necessarily moves up a ladder of increasing perfection (the great chain of being) because of some all-encompassing animating spirit - and you further believe that this process operates at every level of existence, then phyletic evolution (the species level) will mirror ontogenetic "evolution" (i.e. development; the embryonic level). Evolution had, for Haeckel, "proved" that an animating spirit had moved primordial slime upwards to the pinnacle of perfection (humanity) and this process was "obviously" seen also in the development of individual organisms.

Holding fast to these metaphysical commitments made for some pretty weird embryology and some even weirder paleontology. Contemporaries of Haeckel's saw the evidence of the Biogenetic Law everywhere in the fossil record -- Beecher's work on Cambrian trilobites is a case in point.