Thanks for "Embryology of the fossil cephalopods", [Bull. Mus. Comp. Zool. Harvard 3 (1872–4): 59–112].

Regrets error in attributing acceleration concept to E. D. Cope instead of to AH in last edition of Origin, and misrepresentation of their joint view.

If decapod does not pass through zoea stage, is this acceleration? If hypothetical adult retained zoea characters, would this be retardation? Believes obliteration of growth stages frequently due to natural selection. Most interesting points in AH’s letter deal with senile characters. CD attributes them to laws of growth not selection. Explains degraded characters as result of readaptation to simpler conditions. Believes no innate tendency to progressive development exists.

Hopes AH visits F. Hilgendorf’s famous deposit [at Steinheim]. A. Weismann [Einfluss der Isolierung (1872)] makes good use of Hilgendorf’s observations.

Sends copy of last edition of Origin.

Respecting AH’s theory that acceleration of growth produces new characters, urges AH to examine decapods that do and do not pass through zoea stage. Believes there are no marked differences between them.

No need to apologise for not quoting AH’s paper on acceleration and retardation.

Agassiz introduced AH to ammonites and entrusted collection to him. Has followed developmental history of each species and placed them within geological formations. Found evolutionary history of species recapitulated only to a degree in individual development. Stages frequently skipped. Explains why young of later animals are like adults that preceded them. Retardation entirely idea of Edward Drinker Cope. Sends paper to explain it. Acceleration can explain degraded forms. Often like youthful stage with which series began. Often resemble old age of earlier series. Regularity of these series incompatible with natural selection. How can selection account for degraded final stages or for predictability of development? Franz Hilgendorf’s Paludinae from Steinheim lake show same parallelism in development. May be possible to reconcile this with selection. But Trochiformis begins to show degradation in beds where it is most numerous and has largest individuals, i.e., where selection seems to be favouring it. Will work on Steinheim shells this winter.

Discusses his theory of acceleration and retardation of development.