22 PSEUDOCERATITES OF THE CRETACEOUS. 
saddles by the addition of marginals during the later stages of the ontog- 
eny does not occur; the inexact parallelism is produced by the dropping 
out of this stage completely, not by its later and later development. 
Cope, in his great work Origin of the. Fittest, writes as follows:* 
* * * The acceleration in the assumption of acharacter, progressing more rap- 
idly than the same in another character, must soon produce, in a type whose stages were 
once the exact parallel of a permanent lower form, the condition of inexact parallelism. 
As all the more comprehensive groups present this relation to each other, we are com- 
pelled to believe that acceleration has been the principle of their successive evolution 
during the long ages of geologic time. 
Each type has, however, its day of supremacy and perfection of organism, and 
a retrogression in these respects has succeeded. This has, no doubt, followed a law 
the reverse of acceleration, which has been called retardation. By the increasing 
slowness of the growth of the individuals of a genus, and later and later assumption 
of the characters of the latter, they would be successively lost. 
To what power shall we ascribe this acceleration, by which the first begin- 
nings of structure have accumulated to themselves through the long geologic ages 
complication and power, till, from the germ that was scarcely born into a sand 
lance, a human being climbed the complete scale and stood easily the chief of the 
whole? dina 
Acceleration signifies addition to the number of those repetitions during the 
period preceding maturity as compared with the preceding generation, and retarda- 
tion signifies a reduction of the numbers of such repetitions during the same time.? 
Thus, from Cope’s point of view, tachygenesis is the law of progression, 
and retardation is the law of retrogression, and they are both essential parts 
of his law of acceleration and retardation. 
These quotations show that we both have the same conception ot 
the proper use of the word “retardation,” but we differ in the application 
of it. He applied it to such cases as are described here among Pseudo- 
ceratites, whereas I regard these as true arrests of development and not as 
retardations. 
Retardation is exceedingly rare among Ammonitinze, and as a rule in 
other parts of the animal kingdom, and the only examples I am able to cite 
are like those given below in Placenticeras, where the more complex species 
like whitfieldi, that are obviously descended from species like P. syrtale, 
have apparently the nodes and ornaments smaller and developing, as a rule, 
later than in that species, and finally, in extreme forms like those of typical 
whitfieldi, disappearing altogether. 
ap. 142. dP. 182. 
