CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



885 



Stevenson-Hamilton (1912.1, p. 44) observes that 

 the gestation period of the African antelopes is about 

 eight months. Most females seek sheltered spots 

 before calving. About calving time great destruction 

 is wrought by the Carnivora, especially by the hunting 

 dogs (Lycaon pictus), which run down the little ante- 

 lopes with great ease and destroy large numbers of 

 them. Within a very few days of birth all young 

 antelopes and zebras are capable of keeping up in the 

 most surprising manner with the fully grown ani- 

 mals even when the latter are going at full speed. 

 ■ (See p. 846.) 



SELF-EXTINCTION THROUGH ARRESTED VARIATION 



The Italian geologist Brocchi (1814.1, vol. 1), the 

 author of an able work on the fossil shells of the sub- 

 Apennine hills, advanced the hypothesis of some 

 regular, constant law by which species may disappear 

 from the earth gradually and in succession. The 

 death of a species, like the death of an individual, 

 he suggested, may depend upon certain peculiarities 

 of constitution conferred upon the species at its 

 origin, and as the longevity of the individual depends 

 upon a certain force of vitality, which after a period 

 grows weaker and weaker, so the longevity of the 

 species may depend upon the quantity of prolific 

 power originally bestowed upon it; after a season the 

 species may decline in energy and its fecundity may 

 be gradually lessened from century to century. 



Lyell opposed this doctrine on the ground that 

 there is seldom evidence of physiological deterioration 

 in the last representatives of a species. 



Neumayr also opposed this doctrine. He remarks 

 (1889.1, vol. 1, pp. 142-143, 147): 



It has been assumed that species, like individuals, go through 

 a prescribed cj'cle of life, that they arise, flourish, decline, and 

 die, unless they undergo a sort of rejuvenation through gradual 

 variation, and thus extinction has been attributed to the ina- 

 bility to continue to vary. It is undeniable that countless forms 

 have become extinct because they could not adapt themselves 

 rapidly enough to changing conditions, even where these 

 changes covered thousands of years. However, paleontology 

 affords no proof — nor is any evidence to be found elsewhere — 

 for the more extreme view that forms maintain their power to 

 vary for only a limited time, after which they become rigid and 

 unadaptable. That any animal ever ceased to show variations 

 is a purely arbitrary assumption [p. 143]. Nor is the analog}' 

 between decay and death of the individual and of the species 

 or family justified, for senile degeneration and death are by no 

 means peculiar to all living organisms. Among the Protozoa 

 death is the result of external violence, not the necessary out- 

 come and prescribed end of the life cycle, and only among the 

 higher organisms with more complicated methods of repro- 

 duction does it become so. In everj' instance extinction can 

 be explained on the basis of the struggle for existence and 

 without recourse to any mysterious inner causes [p. 147]. 



The idea that self-extinction is caused by an inherent 

 arrest of variation was expressed in another form by 

 Darwin and Wallace, namely, that, as a limitation or 

 cessation of variation would cut off material for 



improvement through selection, a fixed or nonadapt- 

 able type would arise, and extinction would follow. 

 It has been revived or discussed by Doderlein 

 (1888.1), Rosa (1903.1), Abel (1904.1), Plate (1904.1), 

 and Stromer (1905.1). The rdle assigned to the limi- 

 tation of variation (independent of the efJects of envi- 

 ronment) as a cause of extinction again depends upon 

 direct observation as to the modes of evolution. If 

 there is a hereditary progressive trend (such as 

 "Mutationsrichtung") in evolution leading in certain 

 directions, is there also an arrest of such movement? 



Doderlein (1888.1, p. 394) advanced the opinion, 

 especially as a consequence of certain opinions of 

 Cope, that in a long-continued evolution in one direc- 

 tion there is inherited not so much a definite condition 

 as a tendency to continue to develop in that direction. 

 In spite of the law of inertia (Tragheitsgesetz) this 

 inherited tendency continues even if it is no longer 

 useful to the organism unless it is offset by powerful 

 counterforces, such as natural selection under condi- 

 tions of severe competition. Thus organs may arise 

 that are directly harmful to their possessors and may 

 contribute to their destruction, as, for instance, the 

 excessively large canines of the last machaerodont 

 tigers and the gigantic antlers of Cervus eurycerus and 

 C. dicranius. 



Rosa (1903.1) discussed the hypothesis of the pro- 

 gressive reduction of variability and emphasized the 

 fact that highly specialized organisms may show 

 variations but that these variations do not lead to 

 new phyla, or, if they do, only to a slight extent. 



Plate's discussion and criticism (1904.1, pp. 641- 

 655) of Rosa's views is as follows: 



Daniel Rosa in 1899 published a paper [1899.1] attempting to 

 prove that there is a progressive reduction of variability in 

 species, which leads ultimately to their extinction. In the 

 first place, Rosa bases his "law" on the fact that evolution in 

 many cases means a constantly increasing specialization. 

 Species tend to adapt themselves more and more closely to 

 definite environmental conditions and become ever more 

 specialized. The smaller the circle of external factors to which 

 a species is adapted, the more limited becomes its "phylogenetic 

 capacity" (Rosa), the capacity to produce new and distinctly 

 different species. As pointed out by Cope in his "law of the 

 unspecialized," the great phylogenetic lines originated in 

 unspecialized forms with great potential adaptability. But 

 the struggle for existence drives organisms into more and more 

 extreme specialization, thereby diminishing their chances of 

 becoming stem forms of great phylogenetic lines. Therefore 

 one may speak of a "law of progressive specialization." Rosa 

 calls it the "law of progressively diminished variation" and 

 extends its application beyond the cases of very highly special- 

 ized forms. * * * 



However, instances like that of the African elephant, which 

 shows several geographic varieties, go to prove that highly 

 specialized genera do not lose the power to produce new species, 

 although they can obviously riot become the originators of 

 entirely new and divergent groups. Thus specialization limits 

 the "breadth of evolution" (Evolutionsbreite) but does not, 

 as maintained by Rosa, check evolution itself. * * * 



Rosa goes on to set up a law of progressively diminished 

 variability as the cause of progressively diminished variation. 



