886 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



This leads directly to the further assumption that every species 

 approaches its constant form (Rosa, p. 58). Thus he supposes 

 that through inner causes, which we are at present unable to 

 analyze, the power to vary gradually vanishes in the course of 

 evolution, and that every phyletic series leads finally to rigid 

 forms, which, being unequal to any further changes in their 

 environment, are doomed to extinction. * * * 



Rosa's law of progressively diminished variability is based on 

 a fallacy, for, except in cases of parasitism, etc., evolution leads 

 to greater and greater complication, and variability, which in 

 general depends on the number of variable elements, increases 

 as the number of tissues and organs and the number of their 

 characters is augmented. If, as Rosa maintains, variability 

 and the number of variable elements became progressively 

 reduced in the course of evolution, the higher organisms could 

 not have been derived from the lower. 



In high specialization, as for instance, when the primitive 

 mammalian foot is converted into the fossorial extremity of 

 the mole, the saltatorial extremity of the kangaroo, or the curso- 

 rial extremity of the horse, many characters are no doubt lost 

 and variability appears to be diminished, but this is only the 

 negative aspect, and in reality these adaptations are due to 

 the acquisition of new characters by certain structures. * * * 



Rosa conceives of the splitting up of forms into families, 

 genera, and species as a distribution among the descendants of 

 possible variations a, b, c, d, e, . . . z, of every organ of 

 the stem form. Thus one phyletic series will have the possi- 

 bilities a-f, another g-o, a third p-z with respect to the same 

 organ — for instance, the foot. The end result in each case will 

 be rigid forms, one with the foot a, another with h, a third 

 with k, etc. This conception is erroneous. As a matter of 

 fact, the stem form is split up into new species by the acquisition 

 of new characters, but not by a distribution of variations among 

 the various groups of descendants. 



Thus, finally, while Rosa is right in holding that highly spe- 

 cialized species readily become extinct under altered environ- 

 mental conditions, he is mistaken in believing that variability 

 itself is limited by a one-sided organization. Variability has 

 nowhere been wholly wanting; it has mierely worked too slowly 

 and too incompletely at times to prevent extinction. There- 

 fore not only megatheres and ichthyosaurs but countless simple 

 Protozoa, coelenterates, and echinoderms as well have become 

 extinct. 



Simiiarly, C. B. Crampton (1902.1), as cited by 

 C. W. Andrews (1903.1, p. 1), suggests internal causes 

 of extinction as follows: 



In a recent paper by Mr. C. B. Crampton a possible inherent 

 cause of extinction is suggested. It is impossible to do justice 

 to this interesting paper in a short note, but the gist of the 

 argument seems to be as follows: In the original unicellular 

 organism the possibilities of variation are almost infinite, but 

 as soon as evolution along any line begins, these possibilities 

 are restricted, and become more and more so the more highly 

 specialized the animal is; in short, the potential variation of an 

 organism becomes less and less as specialization advances. 

 Furthermore, under the influence of natural selection in each 

 generation the individuals which tend to vary in the same 

 direction will survive, while at the same time, as already 

 pointed out, their capacity for variation becomes more and 

 more restricted. The consequence of this will be that the more 

 highly specialized any stock becomes the more the individuals 

 composing it will come to resemble one another, until at length 

 the same results as arise from close interbreeding, viz, weaken- 

 ing of the stock, and, finally, extinction, may follow. 



Abel, in discussing the whole subject of extinction 

 (1904.1, pp. 739-748), favors the hypothesis of the 



reduction of variabihty as one of the causes of extinc- 

 tion. He observes: 



Even though it wiU never be possible to unravel the ulti- 

 mate causes of the extinction of species, it remains certain 

 that not only external factors but the internal organization as 

 well are to be considered in seeking the ultimate explanation. 

 In most cases probably an excessive, one-sided specialization 

 in combination with a reduction of variability has led to extinc- 

 tion. * * * Much difiiculty has arisen from the confusion 

 of the conception of senility of certain types with that of pre- 

 destined duration or of the restriction of vital energy. Because 

 certain series of extinct animals show signs of degeneration in 

 the last stages of their history it was assumed [for example, by 

 Brocchi] that every species, genus, family, etc., has a prescribed 

 limit of existence, at the end of which the species decays and 

 dies like the individual. The opponents of the hypothesis of 

 progressive reduction of variabihty pointed to the existence 

 of persistent types from the Cambrian to the present time. 

 Whenever we keep in mind the conception of progressive reduc- 

 tion of variability such facts can be interpreted in a new light. 

 Then it is seen that one-sided specialization and associated 

 with it the reduction of variability cause a weakening of the 

 entire constitution and invite extinction, while, on the other 

 hand, conservative, persistent types may live through long 

 periods of time. Furthermore, a review of the past shows that 

 rapidly and richly varying groups die out sooner than slowly 

 developing series. 



Darwin, Haeckel, and Weismann are upholders of the theory 

 of unlimited variability. Haeckel admits that groups that are 

 becoming extinct produce no new varieties. As a matter of 

 fact, a large number of highly specialized forms have died out 

 as the result of their inability to vary sufficiently, and as 

 Wallace [1889.1] pointed out, the possibility of successful 

 adaptation stands in direct relation to the number of favorable 

 variations. However, the number of variations declines as 

 specialization progresses, and it is this restriction of the limits 

 of variation which makes us better able to determine relation- 

 ships of forms near the end of their phylogenetic evolution than 

 in the early stages where variations and mutations are more 

 numerous and the species, which arise in an explosive manner, 

 differ greatly from one another. * * * But not only hyper- 

 trophy of the entire body but hypertrophy of certain organs 

 to a great extent also appears toward the end of phylogenetic 

 series. Very often this may furnish cause for extinction. The 

 question of excessive specialization at the end of a phylogenetic 

 series leads to a consideration of the problem of degeneration 

 or "paracme" [Haeckel, 1906.1, pp. 366, 383]. 



SPECIAL FEATURES OF THE EXTINCTION OF THE 

 TITANOTHERES 



In considering the possible causes of the extinction 

 of the titanotheres and comparing those operating in 

 the case of other mammals we may now sum up the 

 observations which have been more or less fully com- 

 mented upon in this chapter. 



Increase of individual size. — The general uniform 

 increase in individual size in the titanotheres in both 

 the Oligocene and the Eocene epochs indicates that 

 all conditions of life were then remarkably favorable. 

 This is apparently demonstrated by the fact that with 

 one exception the animals in every line of ascent, 

 both Eocene and Oligocene, steadily increased in size. 

 The single exception is the diminutive, supposed 

 aquatic species MetarTiinus jluviatilis of the basal upper 



