consideration of the great powers of increase possessed by all 

 living organisms. It has been calculated, in the case of a small 

 herbaceous plant, about two feet in height, which produces 

 on an average 10,000 seeds annually, that, if all the seeds 

 developed and produced fertile plants, each of which in their 

 turn ripened a similar number of seeds and so on, all the 

 dry land on the earth would be completely occupied 

 by such plants at the end of five years. Many plants, also, 

 produce a greater number of seeds than the example given, 

 and an ordinary Tobacco plant is calculated to produce, on 

 an average, 360,000 seeds annually. That no single species 

 is able to actually monopolise the earth in this way is due 

 to the struggle for existence. 



" While the offspring always exceed the parents in num- 

 ber, generally to an enormous extent, yet the total number 

 of living organisms in the world does not, and cannot, increase 

 year by year. Consequently every year, on the average, as 

 many die as are born, plants as well as animals ; and the 

 majority die premature deaths. * * There is thus a 

 perpetual struggle among them which shall live and which 

 shall die ; and this struggle is tremendously severe, because so 

 few can possibly remain alive."* 



135. It has been noted above that importance 



in many Floras all sub -divisions of the species are indiscri- of s ? b ~ 

 minately termed varieties. Obviously such varieties are by no 

 means all of the same value. We may find that the distinguish- 

 ing characters of a particular variety are always, under 

 varying conditions of existence, transmitted unchanged from 

 the parent to its immediate offspring, that the variety is, 

 in fact, a true sub-species. Since the characters which 

 distinguish a particular sub-species from the rest of its species 

 may include the power of producing a valuable commercial 

 product, immunity from particular forms of injury and 

 disease caused by fungi, insects or other injurious factors such 

 as frost, and so on, a knowledge of such sub-species is 

 obviously of great importance for the Forester, as well as for 

 the Gardener and Farmer. Many races, again, the distinguish- 

 ing characters of which only remain constant under cer- 

 tain conditions of existence and which are therefore of sub- 

 ordinate importance in nature, are of considerable importance 

 in cultivation where the conditions of their existence can be 

 regulated so as to keep their characters constant. The 

 selection of the most suitable sub-species and races for culti- 



* Darwinism by A. R. Wallace, page 11. 



