Natural Selection of Species. 109 



in tiers or stories above the first allowing five feet two inches to the 

 story, in the year 3392 there will be 1,024 stories aggregating a mile in 

 height occupying all the land on earth, every story having a human being 

 on each space of twelve by fourteen inches. For a much longer time than 

 necessary for results like those of the foregoing calculation, the condi- 

 tions of life have been about as they now are, and yet the population of 

 the earth could stand to-day on two Minnesota townships and each individ- 

 ual have a space twelve by twenty inches to stand on. Similar calculations 

 are possible concerning other animal races. Unrestricted, any one of 

 them ought soon to cover the whole earth. Marine life is more prolific 

 than land life. Almost any mollusk or fish increases fast enough to fill 

 the ocean in a few centuries from top to bottom and side to side and 

 crowd the water out of its bed till it covers every continent. And yet 

 we know there is plenty of room in the ocean. The causes that limit 

 the indefinite expansion of the various forms of life have- been in part 

 alluded to. First, the soil and climate which directly limit the range 

 and production of vegetation and indirectly the range of the animal life 

 that depends upon that vegetation. It is further obvious that if two 

 tribes of animals depend upon the same sort ot vegetation for food, it is 

 only a question of time when the two tribes will compete with each other 

 for that food in consequence of their natural increase in numbers. If 

 this competition shall become a struggle for existence the stronger race 

 will exterminate the weaker. The competition is then transferred to 

 varieties and individuals of the surviving race, and the struggle must 

 result in the continued elimination of the weaker members of that race. 

 It is not possible to conceive of any mode of active existence exempt 

 from causes of limitation and consequent struggle for existence. Even 

 the extravagant conception of the human race living on air and building 

 itself above the earth in layers must be limited by the extent of that 

 air. B}' an extension of the calculation we find that in 1,800 years 

 from now the race would cover the land to a hight of sixty-four miles, 

 which is far above any practical atmosphere, and long before the attain- 

 ment of that limit the struggle for life would have to begin even though 

 life depended on nothing but air. But when we consider how many 

 conditions are necessary to any kind of organic life, and how man}' or- 

 ganisms are struggling for the possession of these various conditions, it 

 becomes easy to account for the enormous inroads which are ma<s C 

 the production of each variety by the competition of the rest. Darwin 

 relates that on a piece of ground three feet by two, dug up for the ex- 

 periment, 357 weeds came up, of which 295 were destroj'ed chiefly by 

 slugs and insects. On another spot three feet by four, twenty species 

 of weeds came up, of which nine species were choked out and destroyed 

 by the too rapid growth of the others. Again he estimated that the 



