HOW SPECIES PERISH. 225 



descendants, which most readily adapted themselves to 

 them. In the battle for life in which all living organ- 

 isms from pole to pole, on land and on sea, in earth and 

 in air, are constantly engaged, each individual has to 

 conform most closely to the conditions of its existence 

 or perish. That great weeding and pruning process, the 

 survival of the fittest, is ever at work sorting out the 

 organisms best adapted to their environment, casting 

 aside to die all those not so constituted. On the other 

 hand, the universe is never absolutely in a state of rest ; 

 changes more or less important are constantly in pro- 

 gress. In those parts of the world that have been 

 subject to the greatest amount of change, such as the 

 Polar and Temperate regions which suffer from the 

 periodical disturbances consequent upon glacial epochs, 

 species exhibit perhaps the most important evidence of 

 recent segregation ; whilst in the Equatorial regions 

 which have been exposed to the least disturbance we 

 find some of the most ancient types of creatures, which 

 have preserved their identity comparatively little 

 changed through uncounted ages. In the same manner 

 plants and animals inhabiting the bottom of deep seas, 

 where but little if any of the change taking place on the 

 surface is felt, retain peculiarities of form and structure 

 almost identical through the ages that separate the 

 Lower Silurian, the Cambrian and the Laurentian systems 

 from the Post Pliocene and Recent ones, 



Q 



