402 THE DISTRIBUTION OF ANIMALS AND PLANTS 



climate within comparatively late periods, and since the date 

 of the introduction of many existing species of animals and 

 plants. To this great truth, in some of its bearings, I have 

 endeavoured to direct attention in the previous articles. In 

 the present case it will be necessary to consider these vicis- 

 situdes in their more general aspects, and with some reference 

 to their effects on the distribution of living beings. 



The modern or human period of geology, that in which 

 man and his contemporaries are certainly known to have 

 inhabited the earth, was immediately preceded by an age of 

 climatal refrigeration known as the Glacial or Ice age. This 

 was further characterized not only by a prevalence of cold, 

 unexampled so far as known either before or since, but by 

 immense changes of the relative levels of sea and land, 

 amounting, in some cases, at least, to several thousands of 

 feet. The occurrence of these changes is clearly proved by 

 the undoubted traces of the action of ice, whether land ice or 

 floating ice, on all parts of our continents, half way to the 

 equator, and by the occurrence of sea terraces and modern 

 marine shells at high levels on mountains and table-lands. 

 Perhaps we scarcely realize as we should the stupendous 

 character of the changes involved in the driftage of heavy ice 

 over our continents as far south as the latitude of 40°, in the 

 deposit of boulders on hills several thousands of feet in height, 

 and in the occurrence of shells of species still living in the 

 sea, in beds raised to more than twelve hundred feet above 

 its present level. Yet such changes must have occurred in 

 the latest geological period immediately preceding that in 

 which we live. Proceeding farther back in geological time, 

 we find the still more extraordinary fact that in the middle and 

 earlier Tertiary the northern hemisphere enjoyed a climate 

 so much more mild than that which now prevails, that plants 

 at present confined to temperate latitudes could flourish in 



