54 
PA LEONTOLOG Y: R. R U ED EM A NN 
Proc. N. a. S. 
ADDITIONAL STUDIES IN ARRESTED EVOLUTION 
By RudoIvPh Rukdbmann 
Stat^ Museum, Ai^bany, N. Y. 
Communicated by J. M. Clarke, January 29, 1922 
In a former paper on the "Palaeontology of Arrested Evolution," ^ the 
writer has traced the causes of persistence in animals as seen from the view- 
point of the palaeontologist. 
A number of conclusions as to the distribution of persistent types in 
the different classes and as to the probable causes of persistence were 
there obtained by a statistical method, in which the genera persisting 
through more than two geological periods were utilized. Two entirely 
different groups of such persistent genera were distinguished and these 
were termed persistent radicles and persistent terpiinals. The former 
are primitive central stocks, still adapted to a variety of conditions; 
the latter post-climacteric types which have become fixed in their charac- 
ters by having found stable physical surroundings, or by gerontic condi- 
tions, loss of variability, etc. 
In continuation of these studies the writer has investigated the possible 
influence recorded in fossils, of the different modes of propagation upon 
the persistence of types and has found that all the lower modes of prop- 
agation, viz., propagation by simple division, by budding, by hermaphro- 
dites and by parthenogenesis, are distinctly favorable to persistence, 
mainly through diminution in the frequency and range of variability ^as 
far as it is induced by fertilization. 
In trying to trace the persistence of types to its ultimate causes, beyond 
the processes of heredity, ontogeny, environment and selection, around 
which the life and evolution of animals may be said, with H. F. Osborn. 
to center, it was found that the process of selection may account for 
the cases of persistence where variability has been reduced to a minimum 
by the modes of propagation mentioned before; and that environment 
accounts for resistance in those cases where it has become so stable as 
to lack the actual stimulus for further development or change of form. 
But it is obvious that there are still more important factors involved in 
heredity and ontogeny that make for persistence in organisms, especially 
as that is shown in the post-climacteric forms, or persistent terminals. 
And above all, none of the four factors, selection, environment, heredity 
and ontogeny, gives any clue to the actual mechanics of the processes 
that induce persistence in types. 
Such explanation of persistent types and a clue to the mechanics of the 
processes involved is intimated in the views recently advanced on the 
methods of inheritance and production of new characters by means of the 
genes or character-determiners of the heredity-chromatin. Diirken and 
