Hyatt.] 264 [April 4, 



It is an undoubted fact, as shown by the writer and especially 

 by Barrande and Dr. Branco, that the embryo itself has varied 

 comparatively little throughout time in the Ammonoidea, Nautil 

 oidea, Belemnoidea, and Sepioidea. 



But these statements do not apply to the earliest stages in the 

 evolution of these types. During these earlier stages, when 

 they all branched out from the common stock, the embryos of the 

 Ammonoidea and Nautiloidea became quite different from each 

 other, the embryos of the Belemnoids remained like those of the 

 Ammonoids almost exactly similar to those of the Nautilini as 

 shown by Chalmas and Branco, and finally in the Sepioidea the pro- 

 toconch or embryonic shells changed more completely and soon 

 disappeared. Attention is particularly called to this remarkable 

 fact in the history of the evolution of these forms, that the separa- 

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

 as in the adults near the origin of the orders, and the comparative 

 invariability of the embryo was confined to the subsequent history 

 of these types after separation. 



We have here no space to discuss 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 earlier or concentrated 

 development of ancestral characters, which as we have said follow 

 the same paths, whether progressive and tending to preserve the 

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

 characters of the type. 



We mention the law of concentration of development 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 concen- 

 tration leads to the disappearance of important characteristics 

 often even in short and comparatively small series. It acts fre- 

 quently within a small group like the Arietidae, so that the later 

 larval and adolescent stages are exceedingly unlike the same stages 

 in very nearly related species in the same family. Unless they are 

 willing to take a small well characterized 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 series. 



Slaves of the embryological lamp consider that they must asso- 



