CAUSES or THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



841 



Bateson (1894.1) revived the saltation hypothesis 

 of Darwia as the exclusive mode of the origiri of 

 species. Saltationists, now known as mutationists 

 (of De Vries), refer to Bateson's work as laying the 

 sure foundations for the theory of the origin of species 

 through heritable saltations. At the time Bateson's 

 work appeared it suffered a searching review from 

 Scott (1894.1, pp. 355-374). Matthew (Osborn, 

 1912.372) examined it critically in the light of ver- 

 tebrate paleontology and reached the following 

 conclusions : 



Of the 323 cases of discontinuity cited in mammals the 

 greater part are obviously teratological and have no direct 

 significance in relation to paleontologie evolution except for 

 a very few instances such as the supernumerary or fourth molar 

 teeth of Otocyon. While not significant [in evolution] these 

 teratological cases are interesting because they show the 

 prevalence of homoeosis and indicate that many of the remain- 

 ing cases which might [otherwise] be considered normal salta- 

 tions or reversions may actually be teratologic, but disguised by 

 homoeosis; all of the possibly significant cases (such as the 

 supernumerary molars) are thereby placed under suspicion. 

 Setting aside this suspicion the minority of the "significant" 

 cases in teeth and feet may be said to afford evidence of the 

 meristic variability of vestigial and rudimentary structures. 

 Bateson's statement that such variability is related not to 

 nonfunctionalism but to terminal position in a series appears 

 to me directly in conflict with his [Bateson's] own evidence, 

 as it certainly is with all my experience. This accords with 

 commonly observed data in paleontology, for no paleontologist 

 would question that vestigial teeth or bones are apt to [finally] 

 disappear by "discontinuous" evolution. As to the appearance 

 by salatory evolution of new and primarily functional parts in 

 teeth or feet, I know of no adequate paleontologie evidence in 

 its favor. It is either demonstrably false or decidedly improb- 

 able. In the cases of supernumerary teeth {Otocyon, Myrme- 

 cobius, Cetacea, etc.) saltatory evolution may be regarded as 

 reasonable in default of any paleontologie evidence to the 

 contrary. Meristic or numerical evolution in fully functional 

 vertebrae is intrinsically probable as the only method of evolu- 

 tionary change. 



The fact that so many cases of supernumerary teeth are 

 associated with asymmetry throws doubt on the significance 

 of all such cases; asymmetric variations and those occurring 

 only in upper or only in lower teeth have no analogy in paleon- 

 tology; such cases as occur abnormally are recognized as of a 

 different and nonsignificant class than normal evolutionary 

 changes. 



Summary of Bateson's 323 cases 



The summary of our conclusions is as follows: 

 Of the 323 cases cited by Bateson of discontiuuity 

 in the vertebrae, the teeth, and the skulls of modern 

 mammals, 286 are abnormal, teratological, reversional, 



and have absolutely no significance in evolution. Ten 

 cases of additional fourth molar teeth are possibly of 

 significance, because among the mammals there are a 

 few genera with fourth molars, which probably have 

 arisen by saltation, as in Otocyon and certain African 

 races of Homo. Of aU the cases cited by Bateson 

 there remaia only 27 which may be ranked as prob- 

 ably significant — that is, of the Idnd which may be of 

 actual importance in normal evolution. These are 

 the addition or the reduction of the number of verte- 

 brae ki the spinal column, which is of real significance 

 because of the well-lvnown variations in the vertebral 

 formulae observed among the different genera and 

 even species of mammals, such as Equus caballus 

 (Nordic breed), with six lumbar vertebrae, and E. 

 caballus (Arab breed), with five lumbar vertebrae. 

 The occasional "origin of species" through vertebral 

 saltation is also rendered probable by the fact that 

 vertebrae can be added to or subtracted from the 

 spinal column only discontinuously — that is, through 

 germinal saltation. 



This evidence, in its bearing on the principle that 

 conspicuous major saltations are not a part of the 

 normal evolution of mammals, also throws prima 

 facie doubt upon the less conspicuous, minor salta- 

 tions. It is these saltations which, as stated above, 

 Darwia believed to be among the chief materials out 

 of which adaptations were accumulated through the 

 action of natural selection. The only laiown differ- 

 ence between major and minor saltations is one of 

 degree, not of kind. 



The fact that the vast majority of germinal anoma- 

 lies cited in Bateson's researches have no significance 

 in evolution imder natural conditions, with the excep- 

 tion of the sudden addition or disappearance of com- 

 pletely adaptive organs, like vertebrae and teeth, also 

 throws many minor germinal saltations under sus- 

 picion as material for natural selection, important as 

 they doubtless are in artificial selection and hybridi- 

 zation. This opinion may be contrasted with that of 

 Punnett (1911.1, p. 15): 



Speaking generally, species do not grade gradually from one 

 to the other, but the differences between them are sharp and 

 specific. Whence comes this prevalence of discontinuity if the 

 process by which they have arisen is one of accumulation of 

 minute and almost imperceptible differences? Why are not 

 intermediates of all sorts more abundantly produced in nature 

 than is actually known to be the case? 



The fact is that intermediates are found abundantly 

 in mammalian zoology (see, for example, Osgood, 

 1909.1) as well as in paleontology. 



MENDELIAN DISCONTINUITY IN HEREDITY 



In 1903 the saltation hypothesis was again revived, 

 after the rediscovery of the law of Mendel (1865). The 

 wide prevalence of the separable characters in hered- 

 ity and of the separateness of so-called "imit char- 



