34 



THE WONDERS OF GEOLOGY. Lect. I. 



New Holland contains, as is well known, a most singular 

 assemblage of mammalia, consisting of more than forty 

 species of marsupial animals; of which the kangaroo is a 

 familiar example. The islands of the Pacific Ocean possess 

 no indigenous quadrupeds, except hogs, dogs, rats, and a 

 few bats. 



The distribution of vegetable life, though perhaps more 

 arbitrarily fixed by temperature and by local influences 

 than that of animals, presents many anomalies. From 

 numerous observations, however, it is supposed that vege- 

 table creation took place in different centres, each of which 

 was the focus of a peculiar genus or species; for many plants 

 have a local existence, and vegetate naturally in one district 

 alone; thus the cedar of Lebanon is indigenous on that 

 mountain, but does not grow spontaneously in any other 

 part of the world. It is also ascertained that certain great 

 divisions of the vegetable kingdom are distributed over 

 particular regions : we shall have occasion to refer to this 

 subject in the lecture devoted to the consideration of fossil 

 plants. 



7. Temperature of the earth. — The temperature of 

 the surface of the globe depends on the action of solar light 

 and heat ; hence the difference in the seasons, and climates 

 of various latitudes ; but there are many causes which 

 modify the distribution of the sun's influence, and produce 

 great local variations : under equal circumstances, however, 

 the temperature is found progressively to diminish from 

 the equator to the poles. There is also an internal source 

 of heat, the cause of which has not yet been determined, 

 but is supposed to be connected with the original consti- 

 tution of our planet. It has been ascertained, by careful ex- 

 periments, that below the point to which the solar influence 

 can penetrate, there is, almost everywhere, an invariable 

 increase of temperature, amounting to 1° of Fahrenheit for 

 every 54 feet of vertical depth : it is therefore possible that 



