120 NEW YORK STATE MUSEUM 



the Graptoloidea which became first pseudoplaktonic, being sessile to 

 seaweeds and later free-drifting or floating. Among the Dendroidea, 

 Dictyonema persists from the top of the Cambrian to the Carbonifer- 

 ous, while among the Graptoloidea we meet kaleidoscopic changes 

 throughout the Ordovician and Silurian, the cause of this rapid 

 development having been the departure from the sea bottom and 

 the suspended position of the colonies. 



In general it can be asserted that the sessile mode, if taken up 

 by a whole class, tends to persistence of the types. This inference 

 would, at first glance, seem to disagree with the previous conclu- 

 sion that stability of physical conditions is essential to the per- 

 sistence of types, for sessile forms are largely found in the 

 littoral zone because there the physical conditions most 

 require sessility. But since these same conditions are there also 

 most apt to change, it follows that by becoming sessile, types would 

 seem to expose themselves to early extinction. If the conditions 

 of 'existence change, the organisms can react only in three ways. 

 They either die out or emigrate or adapt themselves. The last 

 reaction leads to new forms. There is hence left only emigration, 

 and this would seem to be the very road to salvation which the 

 sessile forms have closed to them. But it is to be considered 

 here that the principal change of the littoral zone consists in its 

 wandering up and down the continental shelf through the relative 

 movements of the oceans and continents which take place so slowly 

 that the sessile forms, which, moreover, are as a rule provided 

 with very mobile young growth stages, are well able to follow them, 

 as instanced by the persistency of the littoral corals. 



Another factor of the persistence of the sessile forms lies, in 

 our view, in the hard and massive protective covering which most 

 of these types had to develop and which tends to counteract the 

 variability otherwise induced by changing conditions. 



6 A further inference that appears distinctly in the percentage 

 table is that the persistent types, as defined here, prevail in much 

 greater number among the marine forms than among the land and 

 fresh-water animals. This is shown by the 1 1 per cent of per- 

 sistent Pulmonata as against the 30 per cent of the other Gastro- 

 poda and by the scarcity of persistent types among the old classes 

 of Arachnida, Myriapoda and Insecta, the only persistent arach- 

 nids, for instance, being found among the marine Merostomata. 

 This could be a priori inferred from the fact that the climatic and 

 physical conditions in general have changed much more frequently 



