2 76 GENERAL BIOLOGY 



more harsh than that we pursue in making a flower bed. 

 For, do we not sow the seed thickly, to insure a good stand, 

 and then thin out rigidly after germination? Among all 

 organisms the vast majority of offspring are swept away by 

 casualties against which they have no power to cope; by 

 exposure to unwonted conditions, to floods, to drouth, to 

 ruthless enemies, to diseases, etc. Here the elimination is 

 wholesale and indiscriminate. But casualties are more or 

 less local and occasional, and they always leave an excess of 

 young to be eliminated by slower methods which allow some 

 play for the powers or merits of the individual, and, there- 

 fore, some opportunity for competition. 



The struggle for existence. — The thinning out process 

 inevitably goes forward, but it is no longer wholly indis- 

 criminate, for individuals vary. Some are better fitted 

 than others to meet and cope with the perils and exigencies 

 of life. If these be physical agencies, some are better able 

 than others to withstand excess of heat or cold or drouth; 

 if enemies, some are better fitted than others to combat, to > 

 escape or to elude ; if competitors, some are stronger than 

 others and better able to seize and appropriate to themselves 

 the lion's share of the means of livelihood. 



If we did not thin our seedling bed, nature would thin it 

 for us by the slower, but not less certain methods of com- 

 petition; and a few of the seedlings of stronger growth, 

 reaching down more deeply with their roots to the food 

 supply in the soil and spreading out their leaves more broadly 

 to the sunlight would prove the better able to maintain 

 themselves. 



The survival of the fittest. — Herein lies the efficient prin- 

 ciple of natural selection. The fittest survive. Not in the 

 face of casualties; for these sweep out of existence good, bad 

 and indifferent alike. Not in the face of insuperable diffi- 

 culties ; the best seeds may fall where there is not sufficient 



