DEVELOPMENT OF AMMONITES. 



215 



upwards, we see modification after modification begin- 

 ning at the outer part of the Ammonites, and advancing 

 towards the centre of the discs. The innermost convo- 

 lutions often resist these innovations with great per- 

 sistency, so that we usually find upon their surface sev- 

 eral of these states of development in close juxta-position, 

 as the shell of the individual Ammonite begins with 

 the old morphological type, and then adopts the modifi- 

 cations in the same order in which they follow in vast 

 periods in the geological development of the groups con- 

 cerned." 



" The Ammonites," he says moreover, " thus obtain 

 at an advanced and maturer age — only when they have 

 gone through the development inherited from their 

 parents, and as much as possible in the same manner as 

 their parents — the power of modifying themselves in a 

 new direction, that is to say, of adapting themselves to 

 new conditions; yet these modifications may then be- 

 transmitted to the offspring, so as to appear in each 

 subsequent generation a trifle earher, until this phase of 

 development in its turn characterizes the greater portion 

 of the period of growth. But this last, and longest, 

 phase of development scarcely ever sufifers itself to be 

 supplanted by new ones, formed in Hke manner; heredity 

 operates so powerfully, that a period of development thus 

 once predominant, is repeated in the infancy of the 

 Ammonites, even though but slightly indicated. Hence 

 in an individual Ammonite from a recent stratum, the 

 periods of development compressed and forced back 

 upon the innermost convolutions, must appear in the 

 same succession in which they . wrested the dommion 

 from one another. It is extremely interesting to study 

 15 



