THE VARIETY OF FORCE. 403 



It has been already shown that continual changes 

 occurred in the primeval atmosphere and in the prim- 

 eval sea. These changes acting upon animal life 

 produced changes in its composition. For as animals 

 are the result and expression of the conditions under 

 which they are born, it is natural to suppose that 

 when these conditions are changed, the animals should 

 also change. When the conditions of life are abruptly 

 altered and instantaneously transformed, the animals 

 are of course destroyed ; but when, as is usually the 

 case, the changes are gradual, the animals are slowly 

 modified into harmony with the neighbouring condi- 

 tions. The primeval speck of life being acted upon 

 by a variety of forces, became varied in its structure ; 

 and as these forces varied from period to period, the 

 organisms also varied. Complexity of parts results 

 from complexity of environment. Multiformity of 

 circumstance produces multiformity of species. The 

 development of animal life from the homogeneous to 

 the heterogeneous, from the simple to the complex, 

 from uniformity to multiformity, is caused by the de- 

 velopment of the Earth itself from a monotonous 

 water-covered globe with one aspect, one constitution, 

 and one temperature to this varied earth on which we 

 dwell, where each foot of land differs in some respect 

 from the one beside it. The modifications on modifica- 

 tions of the animal are due to the modifications on modi- 

 fications of the medium in which and on which it lived. 

 And this operation of Nature is hastened and facilitated 

 by a law which in itself is murderous and cruel. The 

 Earth is over-populated upon principle. Of the ani- 

 mals that are born, a few only can survive. There 

 is not enough food for all ; nature scrambles what 

 there is among the crowd. If any animal possesses an 



