GENERAL CONCLUSIONS. 23 



rending the animals njion which these fearsome carnivores 

 prey is too well known to need further mention. During 

 the Tertiary period there existed certain carnivores nearly 

 allied to the modern cats, but exhibiting a much greater 

 develojjmeut of the upper pair of tusks, with a corre- 

 sponding reduction of those of the lower jaw. In these 

 creatures, which are known as machiBrodonts, or sabre- 

 toothed tigers, the upper tusks were greatly compressed 

 and flattened, with serrations on one or both cutting-edges ; 

 their length in a species of the approximate size of a tiger 

 being upwards of seven inches. As in the uintatheres, 

 the anterior end of the lower jaw had a descending flange 

 to protect the end of the tusk. Manifestly, such enormous 

 weapons would completely bar the sides of the open 

 mouth, and consequently they could not be used in the 

 manner of the tusks of a lion or tigi-r. It is still more 

 difficult to imagine that these animals could have struck 

 with their tusks projecting from below the closed mouth ; 

 and it would consequently seem that in this instance also 

 the tusks attained a development which was harmful 

 rather than advantageous — this conclusion being confirmed 

 by the fact, for what it is worth, that the sabre-toothed 

 tigers have become totallv extinct, while their less 

 sjjecialized allies continue to flourish. 



Finally, as regards tusks in general, it appears that 

 while in the land carnivores these are always canines, are 

 present in both jaws, and have closed roots, in the other 

 orders of mammals their develoj^ment is apparently some- 

 what capricious, while they are very frequently present in 

 only one jaw — almost invariably the upper — and continue 

 to grow permanently. Moreover, they are almost as 

 frequently incisors as they are canines ; so that tusks 

 ajjparently similar may be in nowise homologous with one 

 another. Never developed to any size in animals with 

 large cranial appendages in the form of antlers or horus, 

 tusks are frequently wanting in those lacking the latter. 

 Primitively their use was undoubtedly as weapons of 

 attack and defence, or to aid in jjrocuring vegetable food ; 

 but in many cases they have subsequently undergone a 

 (frequently sexual) development beyond the needs of such 



