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first one alone being concave, the divided ventral is introduced 

 earlier in' the ontogony and, finally, the division of the outlines by 

 digitations occurs in the earliest neanic substage, replacing the sim- 

 pler sinuous outlines of the preceding suborders. 



In the evolution of a series heredity therefore acts according to a 

 definite law of replacement. The ancestral characters are brought 

 into contact with new adaptive characteristics, which are being con- 

 tinually introduced into the adult and adolescent stages of ontogeny, 

 and these eventually replace the former which are crowded back to 

 make room for them into earlier stages than those at which they first 

 appeared, and in ?na?iy cases the latter are resorbed and disappear 

 during this process. 



It is a fact, as shown by the writer and especially by Barrande 

 and Dr. Branco, that the embryonic shell has varied comparatively 

 little throughout time in the Ammonoidea, Nautiloidea, Belem- 

 noidea and Sepioidea. But these statements do not apply to the 

 earliest times in evolution of these types, when they branched off 

 from the common stock. The embryos of the Ammonoidea and 

 Nautiloidea become quite different from each other, the embryos of 

 the Belemnoids remained like those of the Ammonoids, almost 

 exactly similar to those of the Nautilinidse as shown by Chalmas 

 and Branco, and finally in the Sepioidea, the protoconch or em- 

 bryonic shells changed more completely and soon disappeared. 

 Attention has been already called to this remarkable fact in the 

 history of the evolution of these forms, that the separation of the 

 orders took place rapidly, and in the embryos as well as in the 

 adults near the origin of the orders, and that the comparative 

 invariability of the embryo was confined to the subsequent history 

 of these types after separation. There is also considerable ground 

 for the conclusion that the young, not the earliest stages of shell, 

 are more variable among the degraded types than among progres- 

 sive forms. The facts already stated with regard to the young of 

 Baculites and some crioceran forms show this. 



This paper cannot be devoted to the discussion of the apparent 

 reasons for these changes, but we have been able to explain the 

 mode in which they take place. The mode in each case is the ear- 

 lier or accelerated development of ancestral characters, which as we 

 have said follow the same law, whether progressive and tending to 

 preserve the characters of the type, or retrogressive and tending to 

 destroy the characters of the type. 



