TITAN OTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



only such general adaptive or inadaptive combinations 

 occur as those above cited, but single adaptive or 

 inadaptive organs, such as the brain, the limbs, the 

 feet, the teeth, the horns, have in course of time been 

 contributory causes of survival or of elimination, 

 partly because of direct adaptation or inadaptation, 

 partly because of indirect inadaptation to changes of 

 environment and of life environment. 



Positive evidence. — Simultaneously over large areas 

 of the world extreme specialization, the development 

 of certain dominant characters, has been followed by 

 extinction: large-brained have replaced small-brained 

 phyla; certain types of limb and foot structure have 

 proved fatal to their possessors. This is direct con- 

 firmation of the broad original form of the Darwin- 

 Wallace hypothesis so far as it applies to individuals 

 as a whole and to the groups of individuals known as 

 varieties, races, species. 



Negative evidence. — This evidence, however, does 

 not touch the queston of the survival value of "minute 

 variations in structure, be they ever so slight," to use 

 the Darwin- Wallace language. It does not appear 

 that paleontology or mammalian zoology can give 

 the final answer to this special feature of the Darwin- 

 Wallace hypothesis. It is, however, here pointed out 

 under the heading "Modes and factors of evolution" 

 (sec. 1 of this chapter) that certain fluctuations of 

 proportion may have distinct survival value and tend 

 to be accumulated through natural selection. 



Minute hiocharacters and survival. — It is impossible 

 to determine from paleontology how early in the rise 

 of a new character, such as the minute rectigradations 

 on the grinding teeth, there may be a sufficient sur- 

 vival value, or utihty, to bring the character under the 

 action of selection; opinions differ on this point; we 

 can neither prove nor disprove the survival value of 

 certain minute rectigradations. Our own opinion, 

 incapable as yet of demonstration, is that natural 

 selection may influence the transformation as soon as 

 a number of these biocharacters (either rectigradations 

 or aUometrons) reach such a stage of development as 

 the "mutation of Waagen." It is not impossible that 

 there may be natural selection in these mutation stages- 

 Our opinion is more positive against the hypothesis 

 that selection is the cause of these rectigradations. 



General conclusions. — Our general conclusions as to 

 the part natural selection plays on minute variations, 

 mutations, and fluctuations are negative. It is 

 reasonably certain that under Darwin's general law of 

 selection each strain of individual descent is improved 

 as a whole, but it is still uncertain, largely hypothet- 

 ical, undemonstrated experimentally, how far special 

 selection explains the adaptive evolution of minute 

 separate organs or parts, such as the incipient cusps on 

 the teeth (rectigradations), the incipient rudiments 

 of the horn (rectigradations), the countless minor 

 changes of form and proportion (aUometrons). As 



shown in section 1 of this chapter, these biocharacters 

 seem to be largely orthogenetic in origin and to arise 

 quite independently of natural selection. The con- 

 clusion we have reached is that new rectigradational 

 and proportional characters may become a cause of 

 survival or extinction as soon as they reach a stage of 

 survival value — as soon as they are important enough 

 to cause the life or the death of the animal in compe- 

 tition with other characters. 



SUMMARY OF CONCLUSIONS 



The present aspects of the evidence appear to be as 

 follows : 



Intervariation selection: There is no evidence that 

 minute individual variations, except immunity to 

 disease, have been a cause of the origin or evolution of 

 new characters. 



Interfluctuation selection : There is considerable evi- 

 dence from both paleontology and zoology that the 

 selection of fluctuations of proportion may have been 

 a potent factor in evolution. 



Interrectigradation selection: There is no reason to 

 believe that minute rectigradations, in their most 

 rudimentary stages, have sufficient survival value in 

 competition with other biocharacters to affect the life 

 of an organism. 



Interorgan selection: It is demonstrated that single 

 structures, organs, or functions can certainly be the 

 causes of survival or extinction, especially under con- 

 ditions of stress of environment — biologic or physical. 



Interindividual selection: The chief action of selec- 

 tion between individuals seems to be to standardize — 

 to keep every individual as a whole up to or above the 

 average of adaptability of the group to which it be- 

 longs. Individuals that combine the largest number 

 of favorable, adaptive, and adaptable characters are 

 constantly being selected. In this process organic or 

 coincident selection plays a large part. 



Intergroup selection: By a study of local adaptive 

 radiation of the titanotheres we have discovered many 

 separate lines of descent in which similar biocharacters 

 were evolving at different rates. There has been more 

 or less competition between these groups, varieties, 

 races, and species. Here we observe especially the 

 survival of the unspecialized forms and the extinction 

 of the extremely specialized forms. 



Intergeneric, subfamily, family, and superfamily 

 selection : The principle just stated applies to selection 

 in relation to the survival or elimination of subfamily, 

 family, and superfamily divisions. 



To sum up: Natural selection appears to be oper- 

 ating incessantly and positively to preserve and accu- 

 mulate characters that have survival value and to 

 eliminate all other characters. Numerical reduction 

 within a group and even extinction may be due to a 

 single eliminating cause or to a combination of 

 eliminating causes. 



