188 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



Mastodon longirostris of Eppelsheim, it is very easy 

 to put a limit of species. The Pliocene animal 

 has a chin shortened and deprived of lower tusks, 

 while the Miocene type has two small tusks set in 

 a rather protruded mandible. The mounds of 

 the molars have in the first an alternation marked 

 from one half of the crown to the other half ; in 

 the second they are disposed in an almost perfect 

 transverse line. But this facility of demarcation 

 between the two species is certainly due to the 

 momentary lacuna in the intermediate mutations. 

 We do not yet know of a Mastodon whose lower 

 tusks are exceedingly small, and we may expect to 

 discover this form at the lowest point of the Pliocene 

 formations. But we already know in the last 

 strata of the upper Miocene in the environs of 

 Lyons in particular a Mastodon which comes 

 within the longirostris type by the size of its lower 

 tusks, but whose molars possess the alternate ar- 

 rangement which characterizes the arvernensis type, 

 to such a degree that some isolated molars of this 

 last mutation of the Miocene cannot be distinguished 

 from those of the Pliocene animal. The longirostris 

 and arvernensis Mastodons will probably be one day 

 seen to be united by a series of continuous muta- 

 tions, and all demarcation between the species will 

 then become impossible. 



This continuous connection exists at present 

 between the different Miocene forms of the same 

 branch, from the Mastodon longirostris of the upper 

 Miocene to the pigmy form of the Mastodon an- 

 gustidens which begins in the lower Miocene. 

 In fact, the progressive increase in size is most 



