e, and hold these as its
specific characteristics until old age destroyed them. This skipping is
the highest exemplification, or rather manifestation, of acceleration in
development. In alluding to the history of diseases and inheritance of
characteristics, you in your "Origin of Species" allude to the ordinary
manifestation of acceleration, when you speak of the tendency of
diseases or characteristics to appear at younger periods in the life of
the child than of its parents. This, according to my observations, is
a law, or rather mode, of development, which is applicable to all
characteristics, and in this way it is possible to explain why the young
of later-occurring animals are like the adult stages of those which
preceded them in time. If I am not mistaken you have intimated something
of this sort also in your first edition, but I have not been able to
find it lately. Of course this is a very normal condition of affairs
when a series can be followed in this way, beginning with species a,
then going through species a b to a b c, then a b d or a c d, and then
a d e or simply a e, as it sometimes comes. Very often the acceleration
takes place in two closely connected series, thus:

a--ab--abd--ae---ad

in which one series goes on very regularly, while another lateral
offshoot of a becomes d in the adult. This is an actual case which can
be plainly shown with the specimens in hand, and has been verified in
the collections here. Retardation is entirely Prof. Cope's idea, but I
think also easily traceable. It is the opponent of acceleration, so to
speak, or the opposite or negative of that mode of development. Thus
series may occur in which, either in size or characteristics, they
return to former characteristics; but a better discussion of this point
you will find in the little treatise which I send by the same mail as
this letter, "On Reversions among the Ammonites."

LETTER 254. TO A. HYATT. Down, December 4th, 1872.

I thank you sincerely for your most interesting letter. You refer much
too modestly to your own knowledge and judgment, as you are much better
fitted to throw light on your own difficult problems than I am.

It has quite annoyed me that I do not clearly understand yours and Prof.
Cope's views (254/1. Prof. Cope's views may be gathered from his "Origin
of the Fittest" 1887; in this book (page 41) is reprinted his "Origin
of Genera" from the "Proc. Philadelph. Acad. Nat. Soc." 1868, which was
publis