GENERAL REMARKS. 21 
to retardation of development. A distinction exists between arrestation 
and retardation in development, which is of great importance. Arrest of 
development occurs in many ways. It may take effect locally, as upon the 
sutures, stopping them from developing in complexity of outline beyond a 
certain measure, and thus retaining a semblance to their own young and 
apparently reverting to the condition of the same parts in more ancient 
animals of their own stock. It may take effect upon the whole organism, 
as in Baculites and other uncoiled forms, apparently causing the entire 
animal to revert in its characters to a primitive form. There are innumer- 
able degrees between these two extremes which it is not necessary nor 
appropriate to mention here. Retardation is distinct from these and of 
much rarer occurrence. “ 
The development can not be said to have been retarded in these 
retrogressive forms, since it begins and for a certain period in the ontogeny 
progresses in parallel lines with the ancestors of the group, but passes 
through the modifications more quickly according to the law of tachy- 
genesis. After this its progress is quite suddenly and decidedly arrested, 
and the succeeding stages are no longer parallel with those of their 
ancestral forms. The complexity of the outlines of the lobes and saddles 
in species of Hngonoceras, for example, does increase even in the later 
stages; but this increase is very slight, and the result is a retrogressive 
form that mimics to a certain extent primitive forms among Goniatitinee 
and Ceratitine. Such examples may, nevertheless, belong to the highly 
accelerated class, as is shown by the earlier development in these Pseudo- 
ceratites of the Cretaceous of the three principal saddles that only appear 
in the ephebic stage of the highly progressive forms among Ammonitinz 
of the Jura. Retardation of development certainly does not take place 
in their early stages. Can it be said to take place because they do not 
subsequently proceed to develop numerous marginal lobes and saddles on 
the borders of these same lobes and saddles? 
Retardation of development can mean but one class of phenomena, viz, 
those cases in which a character appears in the ontogeny of descendants 
later than the stage at which it appeared in the life of their ancestors. In 
cases of retrogression such as are noted above, and in all examples of this 
class with which I am acquainted, the complication of the lobes and 
«Bioplastology and related branches of biologie research; Proc. Boston Soc. Nat. Hist., Vol. 
XX VI, p. 79, etc. 
