May 1, 1894.] 



KNOWLEDGE. 



107 



shall find that whereas in the latter the lateral pair of 

 hoofs are considerably smaller than the middle pair and 

 do not touch the ground when the animal is walking on a 

 hard surface, in the former the two pairs are nearly equal 

 in size and are all applied to the ground in 

 walking. In this respect the hippopotamus is 

 the most primitive of all the even -toed hoofed 

 mammals that have survived to the present 

 day, and is, therefore, a creature of special 

 interest to the believer in evolution. It is, in- 

 deed, a member of the great group from which 

 the ruminants are considered to have origi- 

 nated ; although, if the reader should be led 

 from this statement to jump to the conclusion 

 that a hippopotamus was in any sense an an- 

 cestor of the giraffe, he would be led into a 

 grievous error. As is the case with nearly all 

 existing animals of a primitive type, the hippo- 

 potamus, in place of being an ancestral form, 

 is a side-branch from the original stock, which 

 has developed certain specialized features not 

 found in the latter. To show that this is the 

 case, we have but to study the teeth of the various species 

 of hippopotami, which are of such a nature as to show con- 

 clusively that those of the ruminants could not have been 

 derived from them. 



It will be remembered that in the group of animals last 

 mentioned the molar teeth have crescent-shaped columns 

 on their grinding- surfaces ; and those of our readers 

 who recall an article which appeared some years ago in 

 Knowledge under the title of " Teeth and their Varia- 

 tions,"* may recoUect that extinct animals show a complete 



passage from 

 such teeth to a 

 simple type not 

 unlike that now 

 found in the 

 pigs. The molar 

 teeth of the hip- 

 popotami,though 

 of the same 

 general plan as 

 those of the lat- 

 ter, have, how- 

 ever, their four 

 main columns, 

 when partially 

 worn, with a dis- 

 tinctly trefoil- 

 shaped pattern ; 

 and it is quite 

 evident that such 

 a tooth could 

 never have given 

 rise to the crescent-teeth of the ruminants. The hippo- 

 potamus molar is, indeed, quite pecuhar, and its structure is 

 is so well marked and characteristic that any person who has 

 once seen a figure like the one here given could immediately 

 identify any example that might come under his notice. 



Whereas, however, all the members of the genus have 

 molar teeth almost exactly alike, there is a considerable 

 amount of difference with regard to the front or incisor 

 teeth of the different species, and as these differences are 

 of considerable interest from an evolutionary point of view, 

 they may be considered in some detail. 



In the first place, it may be mentioned that all hippo- 



* Eeprinted iu "Phases and Forms of Animal Life." 

 Longmans & Co. 



potami have an enormous pair of curved tusks or canines 

 in each jaw, these being shown on the sides of the two 

 lower jaws represented in our second figure. In the 

 common species, between these huge tusks are two pairs 



Fig. 1. — Molar Tooth of Hippopotamus, 

 showing the trefoil-shaped surfaces on the 

 crown. Actual size. 



Extremity of the Lower Jaw of the Common (a) and Siwalik 

 (b) Hippopotamus. 



of incisors, those of the upper jaw being of nearly equal 

 size, whereas in the lower jaw, where these teeth are 

 cylindrical and project nearly horizontally forwards, the 

 central ones are very much larger than the lateral pair, 

 as shown in Fig. 2 (a). If, however, we examine the lower 

 jaw of the pigmy Liberian species, we shall find that 

 normally there is but a single pair of incisors between the 

 tusks, which would lead to the conclusion that this animal 

 is a more specialized type than its larger relative. The 

 truth of this inference is curiously confirmed by the cir- 

 cumstance that individuals of the Liberian hippopotamus 

 are occasionally met with in which there are two incisor 

 teeth on one side, while on the other there is but the 

 single tooth ; this being an excellent example of what 

 evolutionists term reversion or atavism. This, however, 

 by no means brings us to the end of the variation in the 

 number of these teeth obtaining in the group under con- 

 sideration ; but before proceeding further it is necessary 

 to remark that, since in ordinary mammals the typical or 

 full complement of incisor teeth consists of three pairs, it 

 is natural to suppose that one pair has been lost in the 

 common species. That such is really the case is demon- 

 strated by the extinct Siwalik hippopotamus (H. sivalensis) 

 of the Pliocene deposits of the outer ranges of the Hima- 

 laya, of which the extremity of the lower law is shown in 

 Fig. 2 (b). Here it will be seen that between the two large 

 tusks there are three pairs of incisor teeth, which differ 

 from those of the common species in being all of nearly 

 equal size ; and if we were to examine the upper jaw, we 

 should find that in this also there is the same number of 

 teeth. In the presence of these three pairs of incisors the 

 Siwalik hippopotamus resembles the pig, from which it 

 departs less widely than does the common species in that 

 these teeth are relatively smaller and also of nearly equal 

 size. The Siwalik hippopotamus must accordingly be 

 regarded as a less specialized species than either of its 

 living cousins ; and since, together with an allied species from 

 the Irrawady valley known as the Burmese hippopotamus 

 (H. iravaticus), it is the oldest representative of the genus, 

 its generalized features are precisely what evolutionary 

 considerations would have led us to expect. 



There is, however, yet another curious point in con- 

 nection with these teeth demanding a moment's notice. 

 From the evidence of the two specimens represented in 

 Fig. 2, it is quite impossible to determine which of the 

 three pairs of lower incisors found in the Siwalik 

 hippopotamus have disappeared in the common species. 



