378 SELECTION AND THE STRUGGLE FOR EXISTENCE 



to illustrate this point, are taken from Johansen's experiments 

 with Beans which were self-pollinated : 



Weight of seeds Average weight of seeds 



of parents. of offspring. 



350-400 mg. . 572 mg. 



450-500 . 535 ,, 



500-550 . , . . 570 



550-600 . . . . 565 ,. 



600-650 ,, . . . . 566 ,, 



650-700 . . . . 555 



When dealing with a population consisting of members 

 possessing varied hereditary constitution (i.e. not a pure line), 

 a pure strain can often be obtained by selecting individuals 

 which show a particular desired character and breeding from 

 these, with due precautions against cross-pollination. In this 

 way it might, for example, be possible by artificial selection to 

 obtain pure lines, exhibiting a tall or dwarf habit. Under natural 

 conditions the environment may often exercise such a selective 

 influence (so-called natural selection). The possession of a par- 

 ticular character is sometimes decisive in determining which 

 plant shall survive in competition with others, against adverse 

 conditions of climate, etc. Even in a pure strain, some selective 

 action might lead to the elimination of all but the heaviest, 

 shortest, etc., individuals in each successive generation. This 

 was the essence of Darwin's theory of the origin of new species. 



The majority of plants produce such an abundance of spores 

 or seeds, as the case may be, that even with the most efficient 

 dispersal it would usually be impossible for all the offspring to 

 become established. Many a common plant would, indeed, if 

 the means of dispersal were adequate, and all the progeny of 

 successive generations survived, rapidly cover a large surface 

 of the globe. That this does not happen is due to that ceaseless 

 competition of living organisms with one another which is known 

 as the struggle for existence. A Mullein plant, for example, may 

 produce as many as 700,000 seeds. The resulting seedlings are 

 not of equal vigour, and those first to succumb in the struggle 

 for existence are obviously the weakest. The latter, applying 



