BIG ANIMALS 263 



viduals of that bulk having been observed, the gigantic 

 species to which these teeth belonged must probably have 

 become extinct within a comparatively recent period.' 



If these things are so, the question naturally suggests 

 itself : Why should certain types of animals have attained 

 their greatest size at certain different epochs, and been re- 

 placed at others by equally big animals of wholly unlike 

 sorts ? The answer, I believe, is simply this : Because 

 there is not room and food in the world at any one time 

 for more than a certain relatively small number of gigantic 

 species. Each great group of animals has had successively 

 its rise, its zenith, its decadence, and its dotage ; each at 

 the period of its highest development has produced a con- 

 siderable number of colossal forms ; each has been sup- 

 planted in due time by higher groups of totally different 

 structure, which have killed off their predecessors, not 

 indeed by actual stress of battle, but by irresistible compe- 

 tion for food and prey. The great saurians were thus 

 succeeded by the great mammals, just as the great mammals 

 are themselves in turn being ousted, from the land at least, 

 by the human species. 



Let us look briefly at the succession of big animals in 

 the world, so far as we can follow it from the mutilated and 

 fragmentary record of the geological remains. 



The very earliest existing fossils would lead us to be- 

 lieve what is otherwise quite probable, that life on our 

 planet began with very small forms that it passed at first 

 through a baby stage. The animals of the Cambrian 

 period are almost all small mollusks, star-fishes, sponges, 

 and other simple, primitive types of life. There were as 

 yet no vertebrates of any sort, not even fishes, far less 

 amphibians, reptiles, birds, or mammals. The veritable 

 giants of the Cambrian world were the crustaceans, and 

 especially the trilobites, which, nevertheless, hardly ex- 



