COURSE OF DEVELOPMENT ON THE EARTH. 147 



species or varieties are very few, and differ 

 but little in size) generally contains a great 

 number of species, varying in size as well as 

 in other particulars. Some writers, however, 

 regard very large size as itself an abnormal 

 character in a group. 



A large species must necessarily exist in 

 smaller numbers than a smaller one,' if only 

 because it requires more food, and fewer indi- 

 viduals can obtain food over a given area. A 

 large species, too, is less able to avail itself of 

 shelter, and is consequently more exposed to 

 climatic influences. Again, it is almost always 

 less prolific, and less strong and fierce in pro- 

 portion to its size than smaller species. All 

 these circumstances give large species a certain 

 disadvantage in the struggle for existence ; 

 and hence, when the earth is becoming gra- 

 dually fitted for a higher order of beings, and 

 unfitted for those which formerly possessed 

 it, the large species are (as a rule) the first 

 to disappear, while the smaller species are 

 able, in some cases, to prolong their existence 

 for ages longer. It is probable, too, that as 

 man was contemporary with the huge mammals 



of the Quaternary period, some among them, 



L 2 



