126 CONDITIONS AFFECTING THE 



to germinate six hours before any of the others will, 

 other things being alike, enable it to choke them out 

 altogether. I have shown you that there is no par- 

 ticular in which plants will not vary from each other ; 

 it is quite possible that one of our imaginary plants may 

 vary in such a character as the thickness of the integu- 

 ment of its seeds; it might happen that one of the 

 plants might produce seeds having a thinner integu- 

 ment, and that would enable the seeds of that plant 

 to germinate a little quicker than those of any of the 

 others, and those seeds would most inevitably ex- 

 tinguish the forty-nine times as many that were 

 struggling with them. 



I have put it in this way, but you see the practical 

 result of the process is the same as if some person had 

 nurtured the one and destroyed the other seeds. It does 

 not matter how the variation is produced, so long as it is 

 once allowed to occur. The variation in the plant once 

 fairly started tends to become hereditary and repro- 

 duce itself; the seeds would spread themselves in the 

 same way and take part in the struggle with the forty- 

 nine hundred, or forty-nine thousand, with which they 

 might be exposed. Thus, by degrees, this variety with 

 some slight organic change or modification, must spread 

 itself over the whole surface of the habitable globe, and 

 extirpate or replace the other kinds. That is what is 

 meant by Natural Selection ; that is the kind of argu- 

 ment by which it is perfectly demonstrable that the 

 conditions of existeuce may play exactly the same part 

 for natural varieties as man does for domesticated 

 varieties. No one doubts at all that particular cir- 

 cumstances may be more favourable for one plant 



