400 CONCLUSION 



themselves set seed. It is clear that those which are 

 best equipped for succeeding under the existing con- 

 ditions will be most likely to survive in the struggle 

 to establish themselves. For instance, suppose the 

 place where the seeds fall to the ground is an open 

 spot where heavy rain falls at intervals of several 

 weeks, but that there are very dry periods between : 

 suppose further that the surface layers of soil, say to 

 a depth of 2 or 3 inches, are very permeable to 

 water and easily dry by evaporation, but that the 

 deeper layers are permanently moist. If the radicles 

 of seeds germinating during the rain period grow slowly, 

 so that they have not penetrated more than an inch 

 when the dry period begins, the root will be starved of 

 water and the seedling will die. But if a change occurs 

 in the constitution of the seedling so that the radicle 

 grows quicker and the root gets down to the perma- 

 nently moist layers of soil before the dry period sets 

 in, the seedlings will establish themselves and grow 

 into adult plants. 



To take another case : suppose the seeds to be 

 those of a plant growing on the floor of a forest where 

 the soil is at all times moist, but the light penetrating 

 the foliage of the trees is not more than sufficient for 

 the needs of the seedlings. Suppose the seeds of 

 various species fall on the soil in great numbers, 

 and, the conditions for germination being favourable, 

 nearly all germinate. Here there is no difficulty about 

 the roots there is plenty of moisture in the soil for 

 all. But the seedlings will grow up very crowded and 

 will cut off the light from one another. Those which 

 grow most rapidly will rise above the others, will get 

 all the available light, and will establish themselves ; 

 the slower growing seedlings will be badly shaded and 



