114 THE WORLD OF LIFE 



existed around it, one plant might, in three years only, cover 

 an area equal to 2000 times that of the land-surface of 

 the globe. A close ally of this, Sisymbrium Irio, is said to 

 have sprung up abundantly among the ruins of London after 

 the great fire of 1666. Yet it is not a common plant, and 

 is a doubtful native, only occurring occasionally in English 

 localities. 



Turning to the animal kingdom, we still find the repro- 

 ductive powers always large and often enormous. The 

 slowest breeding of all is the elephant, which is supposed 

 to rear one young one every 10 years ; but, as it lives to more 

 than 100 years, Darwin calculates that in 750 years (a few 

 moments only in the geological history of the earth) each 

 pair would, if all their offspring lived and bred, produce 19 

 millions of elephants. 



The smaller mammals and most birds increase much 

 more rapidly, as many of them produce two or more 

 families every year. The rabbit is one of the most rapid, 

 and Mr. Kearton calculates that, under the most favourable 

 conditions, a single pair might in 4 or 5 years increase to a 

 million. In Australia, being favourable in climate, vegetation, 

 and absence of enemies, they have so multiplied as to become 

 a nuisance and almost a danger, and though their introduction 

 was easy, it has so far been found impossible to get rid of them. 



When the general adaptation of an animal to its whole 

 conditions of life over a large area is favourable, an enormous 

 population can permanently maintain itself in the face of 

 what appear to be dangerous enemies. Two cases illustrate 

 this, and at the same time show how the presence of civilised 

 man leads to their rapid extinction. 



In the eighteenth century the bison ranged over almost 

 the whole of temperate North America, being abundant in 

 Pennsylvania and Kentucky, as well as over the whole of 

 the central plains, while it sometimes extended to the coast 

 of the Atlantic. Within the memory of living persons it 

 abounded west of the Mississippi in countless herds many 

 miles in extent, as vividly described by Catlin the painter, 

 in the stories of Mayne Reid, and in the narratives of 

 numerous travellers and explorers. 



The fact that such a large and rather clumsy animal 



