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 fossil cephalopods, with remarks on the subject. (252/3. The
paper seems to be "On the Parallelism between the Different Stages of
Life in the Individual and those in the Entire Group of the Molluscous
Order Tetrabranchiata," from the "Boston. Soc. Nat. Hist. Mem." I.,
1866-69, page 193. On the back of the paper is written, "I cannot
avoid thinking this paper fanciful.") It seems also that I have quite
misrepresented your joint view; this has vexed me much. I confess that I
have never been able to grasp fully what you wish to show, and I presume
that this must be owing to some dulness on my part...As the case stands,
the law of acceleration and retardation seems to me to be a simple [?]
statement of facts; but the statement, if fully established, would
no doubt be an important step in our knowledge. But I had better say
nothing more on the subject, otherwise I shall perhaps blunder again.
I assure you that I regret much that I have fallen into two such grave
errors.

LETTER 253. A. HYATT TO CHARLES DARWIN.

(253/1. Mr. Hyatt replied in a long letter, of which only a small part
is here given.

Cannstadt bei Stuttgart, November 1872.

The letter with which you have honoured me, bearing the date of October
10th, has just reached here after a voyage to America and back.

I have long had it in mind to write you upon the subject of which you
speak, but have been prevented by a very natural feeling of distrust in
the worthiness and truth of the views which I had to present.

There is certainly no occasion to apologise for not having quoted
my paper. The law of acceleration and retardation of development was
therein used to explain the appearance of other phenomena, and might, as
it did in nearly all cases, easily escape notice.

My relations with Prof. Cope are of the most friendly character; and
although fortunate in publishing a few months ahead, I consider that
this gives me no right to claim anything beyond such an amount
of participation in the discovery, if it may be so called, as the
thoroughness and worth of my work entitles me to...

The collections which I have studied, it will be remembered, are fossils
collected without special reference to the very minute subdivisions,
such as the subdivisions of the Lower or Middle Lias as made by the
German authors, especially Quenstedt and Oppel, but pretty well defined
for the larger divisions in which the species are also well defined.
The condition of the collections as regards names, etc., was chaotic,
localities alone, with some few exceptions, accurate. To put this in
order they were first arranged according to their adult characteristics.
This proving unsatisfactory, I determined to test thoroughly the theory
of evolution by following out the developmental history of each species
and placing them within their formations, Middle or Upper Lias, Oolite
or so, according to the extent to which they represented each other's
characteristics. Thus an adult of simple structure being taken as the
starting-point which we will call a, another species which was a in
its young stage and became b in the adult was placed above it in the
zoological series. By this process I presently found that a, then a b
and a b c, c representing the adult stage, were very often found; but
that practically after passing these two or three stages it did not
often happen that a species was found which was a b c in the young and
then became d in the adult. But on the other hand I very frequently
found one which, while it was a in the young, skipped the stages b and
c and became d while still quite young. Then sometimes, though more
rarely, a species would be found belonging to the same series, which
would be a in the young and with a very faint and fleeting resemblance
to d at a later stage, pass immediately while still quite young to the
more advanced characteristics represented by 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
published separately by the author in 1869, and which we believe to be
his first publication on the subject. In the preface to the "Origin of
the Fittest," page vi, he sums up the chief points in the "Origin
of Genera" under seven heads, of which the following are the most
important:--"First, that development of new characters has been
accomplished by an ACCELERATION or RETARDATION in the growth of the
parts changed...Second, that of EXACT PARALLELISM between the adult of
one individual or set of individuals, and a transitional stage of one or
more other individuals. This doctrine is distinct from that of an exact
parallelism, which had already been stated by von Baer." Th
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