THE EARTH FOR MANKIND 535 



some of the plants will be killed; their struggle for water is 

 unsuccessful. Other plants, with perhaps deeper roots or thicker 

 epidermis, remain alive until a new supply of water arrives. 



No matter what plant or animal we consider, we find that 

 the conditions of life are such that it is impossible for all to sur- 

 vive. There is a struggle for every individual from the very 

 first moment of life. The struggle is along several different 

 lines. ( I ) Every organism is exposed to variations in external 

 conditions (temperature, moisture, light, chemical conditions) 

 in the air, in the water, in the soil. (2) Every organism is sub- 

 ject to attack by other organisms— parasites and predatory 

 enemies. (3) Every organism is in need of food and must 

 somehow get its supplies. It may mean hunting or lying in 

 wait ; it may mean depending upon the wind to bring it to a 

 favorable location, as with spores of fungi or with bacteria ; it 

 may mean actually fighting for it, with members of the same 

 species or with members of other species. In every direction, 

 then, each individual organism is in the midst of a struggle, 

 most factors of which are quite beyond its ability to control in 

 any way ; and of course with most organisms there is no ques- 

 tion of knowing anything about it or doing anything intelligent 

 about it. It is for most organisms (individuals) largely a matter 

 of chance whether they die sooner or later. 



386. The meaning of fitness. jNIost of the new individuals 

 (spores, seeds, eggs) are prevented from reaching maturity. 

 What determines which ones are destroyed all along the road 

 from infancy to maturity, and which ones reach the end of the 

 journey? The expression survival of the fittest summarizes the 

 results ; but, while it is a true expression, it is also misleading. 

 Many of us are apt to conclude that fittest has some moral or 

 religious significance ; but that is not the intention. The fittest 

 rabbit when rabbits are being chased by dogs or foxes is the 

 swiftest rabbit. The fittest rabbit when a severe frost attacks 

 the tribe is the one with the best fur or the one that has stored 

 up the most fat under his skin during the previous summer and 

 autumn. The fittest fish is the one that can dart away quickest 



