still intact, one or the other of them is acting as "layshaft" or, in engineering terms, is "idling."* (1933, pp. 180–181)

If growth and development can be dissociated experimentally, often by such simple stimuli as changes in temperature (Needham, 1933, p. 183), then heterochrony can be easily exploited as a pathway to evolutionary change. Bonner has stressed the importance of disengagement in permitting the flexibility that complex organisms require if they are to change at all. Bonner includes both the independence of organs from each other and the separation of stages in ontogenies of individual structures:

If the steps did not occur in blocks or units that can be shifted or altered in toto without seriously affecting the rest of the organism, evolutionary change in complex organisms might have been virtually impossible. It would have meant that a slight alteration in one small link might have completely disrupted the whole chain, for each step of each part would be coupled into all the steps of neighboring parts. But if there can be separate autonomous units, then all that is presumably needed is to alter the initial cue or stimulus which initiates it; if the cue is moved forward in time relative to the other processes, then the unit as a whole will move forward. (Bonner, 1965, p. 123; see also 1974, p. 164)

And, we might add, produce evolutionary change by heterochrony.

A Metric for Dissociation

The primary desideratum for a study of dissociability would be a satisfactory explanation for it at the cellular level. Such, if I understand an alien field correctly, is not now available. I can only offer in its place, from an area more familiar to me, some questions for a metric to gauge its quantitative effect.

As an illustration, let us consider the simplest dissociation of size and shape. If we can measure the correlation of size and shape in an ancestor, the change of this correlation in descendants will indicate whether heterochrony has occurred and, if it has, will measure both its direction and its magnitude.

Allometry is the study of relationships between size and shape

* Needham was evidently rather pleased with his insight, for he ended his article thus: "The fundamental processes have been envisaged as so many secondary gears engaging or disengaging, as it were, with the primary shaft of basal metabolism. Whatever the value of this analogy may be, there can be no doubt about that of the facts which have been brought together in this review. It might indeed have been entitled, had the author possessed sufficient Kantian audacity, 'Prolegomena to any future theory of the integration of the developing organism'" (1933, pp. 219–220). I will not argue that the idea of dissociability is synthetic a priori.