between size increase and sutural development. The only precondition for such an event is ancestral allometry, a nearly universal property of growth (Gould, 1966). (Isometric ontogenies possess no potential for simple heterochronic change. Allometric ontogenies can be retarded to retain juvenile states or extrapolated into previously unexpressed morphologies.) Moreover, the ammonite suture, for all its apparent complexity, may be the mechanical result of very few generating factors; an increase in complexity may involve no more than an earlier inception or more rapid change in one of these factors-a potentially simple change in gene regulation. When the following three conditions are met, evolution by classic acceleration is almost inevitable:

1. Evolutionary change involves the increasing complexity of a feature already present in ancestors.

2. Ancestral ontogeny is positively allometric for increasing complexity in the feature during growth.

3. Evolution proceeds without lengthening either the ancestral time of maturation or the life span.

Since these conditions are common, evolution by acceleration must be an important path to the development of morphological complexity in specific features.

In summary, three situations generally lead to recapitulation; their widespread occurrence guarantees a high frequency of recapitulation among evolutionary events: (1) the prolonged extension of ancestral ontogeny in size and time—hypermorphosis; (2) a selective advantage for earlier appearance of novelties originally introduced near the end of growth—Stebbins' principle of increasing precocity of gene action; (3) a selective advantage for increased complexity in characters already developing with positive allometry in ancestors.

The Importance of Heterochronic Change: Selected Cases

A PHYLUM: THECAL EVOLUTION IN GRAPTOLITES. Silurian monograptids are colonial animals; their astogeny1 begins with a sicula (Fig. 43) and continues by the progressive budding of individual thecae along a single stipe. Each theca is a complete animal; those nearest the sicula (proximal) are oldest, those furthest away (distal) are youngest. Each theca has its own ontogeny; it begins as a straight tube (simple) which may later bend and acquire a complex aperture (such thecae are termed "elaborate"). Elles (1922, 1923) argued that two recurrent trends marked the evolution of Monograptus (Fig. 44): either elaborate thecae arise proximally and spread distally (the "progressive" sequence), or thecae become simplified distally and spread this alter-