466 Proceedings of the Royal Physical Society. 



(4) That forms that are now isolated in their zoological 

 affinities, and bordering on extinction, are generally highly 

 specialised in some direction, but often show some signs of 

 degeneration, and usually have ancestors of greater specialisa- 

 tion during some former period of dominance. A few% at any 

 rate, seem to show a smaller degree of fertility than might be 

 expected. 



(5) Other forms which have come down to us from a 

 distant period with small amount of change, or with very 

 gradually-acquired specialisation, often show a great resist- 

 ance to death. They are also generally extremely fertile. 



(6) That extinct groups seem almost invariably to have 

 acquired a great degree of specialisation during their period 

 of dominance. 



(7) That the more specialised genera and species of groups 

 tend to have a shorter range in time than the less specialised, 

 although they frequently appear to have temporarily acquired 

 a greater dominance. 



(8) When a group shows very quickly- acquired variation 

 and specialisation, its range is usually very restricted. 



(9) That the later forms in extinct groups frequently show 

 signs of degeneration, and sometimes a more primitive organ- 

 isation than the most specialised forms, possibly owing their 

 persistence to their slower specialisation. 



(10) That long retention of primitive characteristics, or a 

 crreat degree of stability and want of variation, has been 

 usually associated with a long range in time. 



And lastly, higher groups do not spring from the most 

 specialised forms of the parent groups before them in time, 

 but from some more generalised forms in those groups which 

 had retained a more primitive organisation. 



Lowly-organised forms generally show a long geological 

 history, as might be expected. Eadiolaria are known from 

 the earliest deposits, and are found throughout the geological 

 sequence in those rarely-occurring rocks which were laid 

 down under suitable conditions for their preservation. About 

 two-thirds of the two thousand species of Foraminifera that 

 have been described occur in the fossil state, and, according 

 to specialists, the longevity of certain genera, and even 



