402 



HISTORICAL GEOLOGY. 



(smiloddn) necator, x 



ifter 



but other genera have been found much larger, one as 



big as a rhinoceros, and another as big as an ox. The 



saber-toothed tigers were also 

 abundant in South America at 

 this time (Fig. 356). 



Quaternary Mammals of 

 Australia. At the present 

 time the mammals of Australia 

 are all marsupials. So was it 

 also in Quaternary times ; but 

 the species were, again, gigantic. 

 The Diprotodon, for example, 



FIG. 856. Bad of-aiachairodus was a kangaroo as big as a rhi- 

 noceros. Many other gigantic 

 species are aM found. 

 We see, then, that the present distribution of mamma- 



"lian forms was already /established inAoe Quaternary, but 



everywhere the species/ were gigantic 



S6me Important GenerM Questions. 



1. CaXise of the /Cold of thfe GlacialEpoch. The 



intense cold which /characterised rche GMcial epoch may 

 have been due to perres^pial or t\jefosmical causes. It 

 seems right that we-snould, as fad\as possible, account 

 for it by terrestrial causes, and /esolt to the other only 

 if these fail. Now, northern devatwn would probably 

 produce great cold in the nortfkern Aiemisphere. This, 

 then, is certainly a probable cause. But the effect has 

 seemed so great and widespread that many think this 

 cause insufficient, and have therefore looked abroad for 

 extra-terrestrial or for cosmical causes. Among the many 

 causes of this kind which have been proposed, the only 

 one which has attracted much attention is that brought 

 forward by Mr. Croll, which attributes it to slow changes 

 in the form and position of the earth's orbit.* 



* For a discussion of this subject, see " Elements," p. 575. 



