HEEEDITY, VAEIATION 123 



years, Darwin calculates, that in 750 years (a few mouients 

 only in the geological history of the earth) each pair wuiikl, 

 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 ^Ir. Kearton 

 calculates that, under the most favourable conditions, a single 

 pair might in 4 or 5 years increase to a million. Australia, 

 being favourable in climate, vegetation, and absence of ene- 

 mies, 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 con- 

 ditions 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 

 w^hole of temperate Xorth America, being abundant in Penn- 

 sylvania and Kentucky, as well as over the whole of the central 

 plains, while it sometimes extended to the coast of the Atlantic. 

 Within the memor)^ 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 should 

 under natural conditions have occupied so large an area in 

 such vast multitudes, is a sure proof that it had become so 

 perfectly adapted to its whole environment as to effectually 

 protect itself against the numerous enemies that inhabited the 

 same area. Those powerful members of the cat tribe, the 

 jaguar and the puma, would have been quite able to destroy 

 the bison had it not been protected by its social instinct and 

 high intelligence. The wolves which hunt in packs, and are 



