274 GENERALIZATIONS. 



be regarded as the dawn of the existing creation. It is an- 

 nounced by an abundant development of leafy trees (generally 

 Dicotyledons) and of Mammalia. With the Eocene formation 

 also began the distribution in zones of heat and life. The earth, 

 and probably the whole solar system, in that epoch passed 

 through a higher phase of their development. The conditions 

 of terrestrial temperature were determined by the position of 

 'the globe with regard to the sun, and by the state of temperature 

 in the regions of space. During earlier geological periods influ- 

 ences acted which had been connected either with the wide ex- 

 pansion of the solar mass or with the heat belonging to the earth 

 itself. Temperature was, at all times, modified on the globe 

 partly by the distribution of land and sea, and partly by the 

 powerful agency of marine currents, which depended on the 

 relative position of sea and land. 



Part II. ORGANIC NATURE. 



Looking back at the succession both of plants and animals in 

 the various ages of the earth's history, we learn from numerous 

 recent investigations that, at the limits of the geological periods, 

 the species of animals and plants are as it were interlaced ; and 

 there is an analogy between these periods and the zones into 

 which mountains are divided, to represent their characteristics 

 at different elevations. Thus the higher boundary of the zone of 

 trees indicates the spots where the forest ceases ; but in favour- 

 able positions some trees may still be met with at a greater 

 height. The zone of snow is at first represented by a few specks, 

 and on going up the mountain we arrive at the fields of granular 

 snow spreading far and wide. In like manner, geological periods 

 have no sharp line of demarcation ; they indicate the principal 

 divisions of a continuous development, the limits of which are 

 placed at those periods in which the greatest transformations 

 were effected. 



Organisms of different epochs are intimately connected. 

 Species are found common to two consecutive periods ; and the 

 whole of organic nature, from its primaeval commencement to 

 the present day, has been altogether in perfect harmony. 



A gradual approximation is manifest, in the organized world, 



