REPORT OF THE DIRECTOR I916 120, 



Living under the normal conditions, which are fixed and vary only very 

 slightly in all parts of the world, we find the vast majority of phylogenetically 

 new forms with a few still vigorous less young types ; under the conditions 

 grouped in the second heading, which show an almost infinite variation, we 

 find the vast majority of all the older forms still persisting, and a very few 

 of the more recent types. 



If we are ever to find recent representatives of such past types as the 

 cystideans, blastoids, trilobites or eurypterids we shall find them living under 

 abnormal conditions to which they became adapted when at the height of 

 their vigor and from which no subsequently developed competing type has 

 succeeded in ousting them. We must lock for them in the deep sea, in fresh, 

 very saline, acid or alkaline water, in regions of great cold, great heat or 

 great climatic change, or in some such situation far removed from the opti- 

 mum conditions under which marine life is maintained. It is probable, how- 

 ever, that none of these types were possessed of such vigor that they were 

 able to colonize localities of such a nature that they could not be reached by 

 competing forms of later development or with a subsequent period of maxi- 

 mum vigor. 



Summary of Factors of Persistence 



In summing up, our evidence appears to show that the lower 

 classes and within each class again, the lower divisions tend to 

 have more persistent types than the higher ones, or in other words, 

 that the groups that have been overtaken by their more rapidly 

 advancing relatives are apt to become stagnant and persistent; 

 as a corollary, persistent types appear more frequently after the 

 climacteric period of a group than before or during the same ; in 

 some cases, however, they clearly form the central vigorous stock 

 from which the shorter lived types branch off. In such cases 

 they are primitive types or forms that vary little from the general 

 class-type in contrast with the aberrant and specialized types which 

 as a rule are short lived, however well adapted they may be to 

 special conditions. Sessile classes, further, show a greater number 

 of persistent types than vagile forms ; the persistent types prevail 

 more among the marine forms than among the terrestrial or fluvia- 

 tile types. Isolation, as by a subterranean habitat was further recog- 

 nized as favoring longevity of types. On the continents the 

 absence of transgressions of the sea and orogenic movements 

 through long stretches of geologic periods have been favorable to 

 persistence of types (asylums). As factors contributory to generic 

 longevity were further mentioned small and inconspicuous size, 

 individual vitality, production of large broods, restriction to most 

 easily procurable food. In some cases the early development of 

 superior offensive and defensive arms appears to have been of 

 great effect in making the race conservative. 

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