ANIMAL PEDIGREES 207 



part, but is coiled spirally like the majority of 

 Foraminifera. The central or oldest turns of this 

 spiral are not chambered ; the outer turns are 

 divided by partitions into chambers, and these 

 chambers, as we follow the spiral round, become 

 wider and wider, so as to overlap and wrap round 

 the older part of the shell, at first partially but 

 ultimately completely ; the first chamber that com- 

 pletely surrounds the shell marking the transition 

 from the spiral to the discoidal type. 



We thus find that the discoidal Orbitolite shell 

 commences its development as a spiral shell, and 

 acquires the discoidal character merely through an 

 exaggerated mode of growth on the part of the 

 spiral. The Recapitulation Theory tells us that 

 this is to be interpreted as meaning that the dis- 

 coidal shells are descended from spiral ancestors, 

 and the close agreement between a young Orbito- 

 lite and an adult Peneroplis suggests that either 

 Peneroplis itself, or forms closely allied to it, were 

 the actual ancestors. 



The Orbitolite is peculiarly instructive, owing to 

 the fact that the addition of new chambers during 

 growth takes place in such a way as to leave the 

 older parts of the shell unaltered and fully exposed 

 to view, so that simple inspection of an adult shell 

 reveals the whole course of development, and shows 

 us not merely the anatomy but the embryology as 

 well. It is as though a kitten were to develop 

 into a cat, not by interstitial growth in all its parts, 

 but by the addition of successive lengths to its 

 nose, its ears, its legs, and its tail; the additions 



