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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 Arietidae, 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. As a 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. 



Evolution is apparently a mechanical process in which the action 



