65 



CHAPTER IX. 



Darwin's section of the joint memoir read 



BEFORE THE LINNEAN SOCIETY JULY 1, 1858. 



The first section of Darwin's communication consisted 

 of extracts from the Second Chapter of the First 

 Part of his manuscript essay of 1844. The Part was 

 entitled " The Variation of Organic Beings under 

 Domestication, and in their Natural State," and the 

 Second Chapter was headed "On the Variation of 

 Organic Beings in a State of Nature ; on the Natural 

 Means of Selection ; on the Comparison of Domestic 

 Races and True Species." The extracts first deal 

 with the tendency towards rapid multiplication and 

 the consequent struggle for life. The average constancy 

 of the numbers of individuals is traced to the average 

 constancy of the amount of food, "whereas the 

 increase of all organisms tends to be geometrical/' 

 Practical illustrations are given in the enormous 

 increase of the mice in La Plata during the drought 

 which killed millions of cattle, and in the well-known 

 and rapid increase of the animals and plants intro- 

 duced by man into a new and favourable country. 



The checks which operate when the country is 

 stocked and the species reaches its average are most 

 difficult to detect, but none the less certain. If any 

 check is lightened in the case of any organism it will 



