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Attention is given to the acceleration of development because it will 
be used in this paper and also because in looking at the young in the 
usual haphazard way, naturalists often do not find the strong marks 
of affinity which the ordinary modes of studying lead them to 
anticipate. The law of acceleration explains the disappearance of 
important characteristics which often occur even in short and com- 
paratively small series. It acts frequently within a small group like 
the Arietide, so that the later larval and adolescent stages are 
unlike the same stages in very nearly related species in the same 
family. Unless investigators are willing to take a small well-char- 
acterized group and follow out all its transformations they cannot 
hope even to understand the remarkable phenomena which are 
shown more or less in the history of every complete genetic series. 
Embryologists generally consider it essential to associate all 
_ forms having similar embryos, and to place widely apart in classifica- 
tion all forms having different embryos. Asa matter of experience 
that is correct, but it does not apply to the earliest times in the 
evolution of types and the surest guides of affinity are sometimes 
the adult gradations of forms. These show that the Nautiloidea and 
Ammonoidea with comparatively distinct embryos are nevertheless 
more closely related than the Belemnoidea and Ammonoidea which 
have precisely similar embryos, and Sepioidea and Belemnoidea 
which have very distinct embryos must also be affiliated. 
The embryos of all these must have been precisely similar at 
their origin, but they afterward became varied in the different 
orders, and we cannot lay down any hard and fast rule by which 
the embryo becomes an invariable criterion of affinity. We think 
there is ample reason in the structures of these shells themselves to 
account for the embryonic differences, and that it is possible to 
reconcile them with the affinities indicated by the gradations 
observed between the adults. These reasons which we have only 
space to allude to here consist in tracing the gradations of adult 
structures and in a series of correlations which are plainly apparent 
between the adult structures, and the habits of the animals, and the 
power which habits in conjunction with effort have to change the 
adult structures, and then by the action of the law of acceleration 
in heredity to change even the embryos, either quickly, when the 
habits are widely changed, or more slowly when they vary but 
slightly with the progress of time. 
laps is apparently a mechanical process in which the action 
‘ Ee 
