128 NEW YORK STATE MUSEUM 



greatly helped to give the group stability ; ' in others, the limulids, 

 the burrowing habit combined with an excellent leathery defensive 

 armor have allowed the little offensive and sluggish animals to 

 persist through an astonishing length of time. 



The whole class of turtles which, in contrast to the Dinocerata, 

 still flourishes today, although already arising in the Triassic, is 

 an example of a group that has successfully specialized for protec- 

 tion and thereby survived, while the Dinocerata, in spite or in 

 part because of their gigantic dimensions, are extinct (Wieland). 



Persistent Types Originally the Most Vigorous Stocks 

 We have seen that a great number of persistent types and relicts 

 have found isolation in restriction to regions with a minimum of 

 ecological competition, that is, by withdrawing to greater oceanic 

 depths, to a burrowing habit, to underground water on land, to 

 islands, to lakes and old rivers of ancient continents (as Ceratodus 

 in Australia and some fishes of Africa), to very saline water, to a 

 nocturnal habit, etc. All these persistent forms are to be considered 

 as senescent types which are in a very delicately balanced condi- 

 tion and would quickly succumb if younger and more vigorous 

 forms should intrude upon their ecological territory. While they 

 thus now appear as weak forms, they ivere, however, originally the 

 most vigorous stocks which were able to reach and adapt them- 

 selves to the more abnormal conditions at the periphery of the 

 ecological field. It is not probable that a mature or senescent 

 form can be forced into new territory; when subjected to competi- 

 tion by younger and more vigorous species it merely dies away. 

 This relation of the old types to the younger types still in process 

 of evolution has been clearly recognized by Austin H. Clark (op. 

 cit. p. I36ff) from his studies of recent crinoids. He writes: 



It must be remembered that, although we are at present concerned only 

 with deep-sea forms, internal specific pressure due to enormous increase in 

 the numbers of individuals within a species operates not only to cause a 

 species to colonize bathymetrically undesirable locations, or unnaturally cold 

 and uncongenial regions such as the polar seas, but also to force species into 

 small localized areas, or as it were pockets, possessing special and circum- 

 scribed unfavorable characteristics where they may be able to remain unin- 

 fluenced by the changes in the general fauna of which they once formed an 

 integral part. Thus we find such genera as Artemia, and in fact all the 

 phyllopod crustaceans, Carcinoscorpius, Tachypleus, Xiphosura (collectively 

 known to palaeontologists as "Limulus"), Lingula, and many others occupy- 

 ing special areas into which no competing younger forms have ever forced 

 themselves. . . . 



