THE ARGUMENT FROM EMBRYOLOGY 91 



ever, in its early stages of development the Limpet 

 has a spiral shell, which is lost on the formation of 

 the conical shell of the adult. 



Recapitulation is not confined to the higher 

 groups of animals, and good examples are found 

 among the Protozoa. One of the best instances is 

 that of Orbitolites, one of the most complex of the 

 Foraminifera, which, during its own growth and 

 development, passes through the series of changes 

 by which the discoidal type of shell is derived from 

 the simpler spiral shell. This forms an instructive 

 example, for, owing to the mode of growth by 

 addition of new shelly matter, the older parts are 

 retained often unaltered, and in favourable examples 

 all stages can be determined by simple inspection of 

 the adult shell. (Fig. 14.) 



The mode of growth of shells is important, since 

 it gives an opportunity for comparing the ftalceon- 

 tologicat and embryological records. In such a shell 

 as that of the Nautilus the central chamber is the 

 oldest and first formed one, to which the other 

 chambers are added in succession. If then the 

 development of the shell is a recapitulation of 

 ancestral history, the central chamber should repre- 

 sent the palaeontologically oldest form, and the 

 remaining chambers in succession forms of more 

 and more recent origin. 



In the shells of Ammonites it has been shown 

 that such a correspondence between historic and 

 embryonic development really exists. In the middle 

 Jurassic deposits the older Ammonites are flattened 

 and disc-like, with numerous ribs ; in later forms the 



