124 NEW YORK STATE MUSEUM 



8 Many persistent types whose geologic history is well traceable 

 in the rocks show a slow development, a distinct climacteric period 

 and a long postclimacteric period. Good examples of such genera 

 are the bryozoan Berenicea which is " rare in Ordovician, very 

 abundant in Jurassic and Cretaceous, less frequent in Tertiary and 

 Recent " ; the coral Halysites which begins in the Ordovician, 

 culminates in the Silurian and extends into the Lower Devonian ; 

 the echinoid Cidaris which appears in the Permian, is very abund- 

 ant and variable in its climacteric period in the Jurassic and Cre- 

 taceous and persists until today; the worm Serpula, which appears 

 in the Silurian, culminates in the Mesozoic and still persists ; also 

 the Paleozoic representatives of Lingula, which begin in the 

 Ordovician, culminate in the Silurian and Devonian, and decline in 

 the Carboniferous and Permian ; the ostracod Bairdia which 

 develops in the Ordovician, reaches its maximum development in 

 the Carboniferous and then continues in diminished numbers until 

 the present day. The diagram of such slowly developing long- 

 range genera would appear like this : 



We believe that the life history of most long-lived genera 

 when fully known will be found to have possessed this mode of 

 development, in contrast to all rapidly developing genera which as 

 a rule disappear as quickly, either by changing through their 

 variability into new genera or by extinction. This contrast is well 

 shown in the Cephalopoda by the difference between the slowly 

 developing Nautiloidea with their fair number of persistent genera 

 in a relatively small number of genera (12 in 170) and the 

 Ammonoidea, which are so variable that they count 452 genera 

 and change so rapidly that they have no persistent forms. 



Connected with the preceding factor of persistence in genera is 

 the following fact : that the persistent genera which slowly develop 

 never produce many species during a single geological period. 

 Genera that produce a great number of species, as a rule become 

 soon extinct by progressive development to new generic groups 

 and a crowding out of the older primitive stock by the descendants. 



9 There are a number of minor factors that pertain to great 

 longevity in the organic world and that are clearly present in some 

 of the persistent genera, (a) One of these is an extreme individual 



found, as, for example, Proteus the subterranean caves of Carinthia in 

 x\ustria, Siredon the land-locked lake§ about Mexico City, Cryptobranchus 

 the volcanic lakes of Japan, and the others the asylums cited by Suess. 



