CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



835 



2. Internal initiation and causation (Darwinism): 

 Chance, accidental, fortuitous heritable variations in 

 the germ, minor germinal saltations, germinal muta- 

 tions (De Vries), predispositions that give rise to 

 favorable or unfavorable alterations of character and 

 function, of survival value, accumulated under the 

 action of natural selection or natural elimination. This 

 is the pure Darwinian interpretation. 



Evidence : According to this theory variations should 

 originate no adaptive tendency except through the 

 cumulative action of selection in a given direction; 

 in their first appearance variations should be indefinite, 

 indeterminate, and should trend in all directions. 



3. Internal-external initiation and causation (tetra- 

 kinesis) : According to this theory evolution is neither 

 chiefly external (environmental, ontogenetic) nor inter- 

 nal (germinal) but is due to the combined action of 

 internal and external causes accumulated by natural 

 selection. This theory (Osborn), which is based upon 

 the data of paleontology, experimental zoology, and 

 physiology, contains certain elements of both the 

 Lamarckian and the Darwinian interpretations, 

 although it is unlike either. This may be referred 

 to as the principle of the inseparable action of four 

 factors in development (Osborn, 1908.308), as the 

 theory of tetrakinesis in evolution, or as the theory 

 of the action, reaction, and interaction of four com- 

 plexes of energy under the continuous operation of 

 natural selection (Osborn, 1917.463). 



Evidence: According to this interpretation variation 

 should move chiefly along definite lines; it should be 

 orthogenetic; but, unlike the procedure hypothesized 

 by Lamarckian interpretation, variations may or may 

 not be preceded by similar bodily modifications. 



ANALYSIS OF THE EVIDENCE ON THE MODES OF ORI- 

 GIN OF VARIATION AS CONSIDERED IN DARWINISM, 

 LAMARCKISM, AND TETRAKINESIS 



It is obvious that sharp, clear, unbiased observation 

 and analysis of the modes of origin of variations may 

 have a crucial bearing on the choice between the three 

 theories stated above, of the causes of evolution. 

 For example: 



1. If in their germinal variations — rectigradations 

 and allometrons — mammals are observed invariably 

 to follow antecedent bodily (somatic) and environ- 

 mental modifications, such modes of origin would 

 constitute strong evidence for the Lamarckian theory. 

 It will be pointed out, however, that germinal varia- 

 tions do not follow antecedent bodily modifications 

 (somations) as a rule but only in certain instances. 

 These exceptions cast strong doubt on the pure 

 Lamarckian interpretation. 



2. If new biocharacters (rectigradations) are her- 

 alded in indefinite, indeterminate, accidental varia- 

 tions, by the rudiments of new, spontaneously ap- 

 pearing characters at various points in the teeth and 



101059— 29— VOL 2 10 



skeleton, if from these unstable origins stability and 

 adaptation are gradually attained by the survival of 

 certain characters and the elimination of others, such 

 modes of origin would constitute strong presumptive 

 evidence in favor of the Darwinian interpretation — 

 namely, that the adaptive arises by selection out of 

 fortuitous variations. If, on the contrary, minor 

 organs, saltations, fluctuations, sports, are observed 

 to have no relation to the main trend of adaptation, 

 this would tend to show that the Darwinian hypothesis 

 affords an inadequate explanation of the evolution 

 of the titanothere. 



3. If the modes of variation we have been observing 

 in the titanotheres — rectigradations and allometrons — 

 are in the main definite, determinate, and generally 

 adaptive in direction but do not invariably follow the 

 direction of individual somatic modifications, there 

 must be some cause or complex of causes other 

 than those afforded exclusively by either the pure 

 Lamarckian of the pure Darwinian hypothesis. 

 These rectigradations and allometrons are the modes 

 of variation, observed in the titanotheres and other 

 mammals, which have led Osborn to propose and to 

 develop the theory originally termed (Osborn, 1908. 

 308) "the four inseparable factors of evolution," 

 more recently termed "the theory of tetrakinesis" 

 (Osborn, 1917.462). 



OBSERVED PRINCIPLE OF TETRAPLASTIC DEVELOPMENT 

 OF BODY form; THEORETIC PRINCIPLE OF THE 

 TETRAKINETIC EVOLUTION OF THE GERM 



Basis of the tetraJcinetic theory. — The tetraldnetic 

 theory,- which has been developed gradually by the 

 author between the years 1893 and 1917,^' is based 

 upon the modern conception that the visible bodily 

 evolution of the titanotheres is the result of the in- 

 visible germinal evolution of the titanotheres; conse- 

 quently that the modes of bodily evolution reflect 

 the modes of germinal evolution. If the modes of 

 bodily evolution are orderly, determinate, generally 

 adaptive in direction (entirely distinct from the trial 

 and error modes of pure Darwinism), there must be 

 causes of this orderly evolution of the germ. The 

 conclusion is reached that these causes are not purely 

 internal, germinal, nor purely external, environmental, 

 somatic; that they are in some manner internal- 

 external. 



The fourfold principle of development. — It is observed 

 that in the development of its bodily form and func- 

 tion each titanothere is tetraplastic in the sense that 

 from the stage of the fertilized germ to adult age it is 

 plastically molded by the action, reaction, and inter- 

 action of four centers of influence, namely, that con- 

 tained in the heredity germ, that found in the 

 developing organism, that of the life environment, 



Q2 The special papers and researches in which this principle and theory are treated 

 are listed in the bibliography at the end of this chapter. 



