BIOLOGICAL PALAEONTOLOGY in 



group of organisms is approaching its acme. This out- 

 break of morphogenetic activity rarely, if ever, equals 

 the earlier phase in energy or scope. Differentiation of 

 genera and species, rather than families or orders, marks 

 its incidence. Since the acmaic stage is usually one in 

 which the various members of a group excel in numerical 

 wealth, it may well be that the separation of relatively 

 small sections is an expression of the abundance of 

 individual variation available for selection. Most genera 

 or species evolved at the acme of a group are short- 

 lived, though in their brief careers they may attain 

 remarkable success. The types that survive to follow 

 the descent of the group from its acme are usually 

 those that took the least prominent part in acmaic 

 specialization. 



The third, and final, outburst of evolutional activity 

 occurs in the gerontic phase of a stock. Differentiation 

 of ordinal quality is rarely developed at this stage, but 

 new, often surprisingly aberrant, families may appear. 

 Modifications of such importance that a rejuvenated 

 stock is produced may occasionally be attained. An 

 example of this phenomenon is probably afforded, 

 among Vertebrata, by the Ophidia, which seem to range 

 back no further than the Cretaceous period, when 

 Reptiles as a class were on the wane. Perhaps the 

 Sepioids are similarly related to the Belemnoids, and the 

 Clypeastroids to the Holectypoids. Usually, however, 

 the families that branch from a gerontic stock show 

 many features of racial old age even at their inception, 

 and sink into oblivion together with their more 

 orthodox associates. 



The Brachiopoda may be cited afresh in illustration 

 of the three stages of rapid specialization. The Telo- 

 trematous superfamily Spiriferacea, whose relatively 

 brief career lasted from Ordovician to Jurassic times, 



