194 GEOLOGICAL BIOLOGY. 



varieties, perpetuate the varieties, or can cause a race to 

 grow up in which the varietal characters shall become relatively- 

 permanent. Numerous facts of this kind are familiar, as our 

 common breeds of horses, cows, pigs, domestic pigeons, 

 flowers, fruits, etc. As illustrative of the extreme modifica- 

 tion possible, the greyhound and the pug-dog may be cited. 



3. Darwin further observed that varieties occur under 

 natural conditions ; that there are doubtful species, or forms 

 which are intermediate between the typical species. 



4. He found by an analysis of the plants of twelve coun- 

 tries, and the coleopterous insects of two districts (and this 

 result was confirmed by later study), that the larger genera 

 present the greater number of varieties and are the more widely 

 distributed. 



5. The natural increase of organisms by generation is 

 vastly in excess of the actual number reaching maturity ; in- 

 crease is by geometric ratio, but the increase of adults is, at 

 best, only a very slight aritJnnctical ratio. Linne showed that 

 from a single plant producing only two seeds, if all the seed- 

 lings were to live, in twenty years there would arise a million 

 plants. Darwin estimated that from a single pair of elephants 

 breeding at the age of thirty years, and continuing breeding 

 until ninety years old, producing three pairs of young in the in- 

 terval, at the end of the fifth century there would arise fifteen 

 million elephants alive at one time descended from the first 

 pair. 



6. There are innumerable checks to increase, as nature of 

 climate and of food, but particularly mutual checks, as strug- 

 gling of individuals for the same food, or for the same set of 

 favorable conditions. This is the general law of struggle for 

 existence. 



7. Natural Selection. — Darwin then argued that the con- 

 ditions of environment ; the abundance of food, or lack of it ; 

 the favoring climate, or the opposite ; the accessibility of 

 food, or difificulties in the way of obtaining it ; would all work 

 together and separately, as either favorable or unfavorable 

 conditions for each individual, according as he was more 

 poorly or better adapted to live under them than his fellow ; 

 that each of the characters of a varietal nature must have 



