long taproot by the radish, or the dropping of leaves in the autumn. The 

 struggle is a continuous activity at every stage of life. It is an overcoming of 

 obstacles and resistances which arise from the changing environment and from 

 the activities of other living things. It goes on even where there are no 

 enemies or rivals, and even where the needed food, water, air and minerals 

 are abundant. Life is itself aggressive, and all its processes are attacks upon the 

 outer world — or resistance to attacks from that outer world. 



The Meaning of Fitness From the fact that more individuals are born 

 than can possibly survive comes the pressure of population. Only a small 

 fraction of those born will live long enough to reproduce themselves. But 

 which one will die at this stage, or the next? Which ones will complete the 

 cycle? The elimination which goes on in the struggle has been called the "sur- 

 vival of the fittest". This expression is quite misleading, for it suggests some 

 absolute quality, a general superiority that is important in itself. But as we 

 have seen (p. 467), the intention of Darwin and of others was to describe some- 

 thing more directly related to a specific situation. Thus the fittest rabbit 

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

 when a severe frost attacks the tribe, the fittest rabbit is the one with the best 

 fur, or the one that has stored up the most fat under the skin during the pre- 

 vious summer and autumn. There is no absolute standard for plants and 

 animals. Fit7iess is a relationship between the organism and all the features of its 

 surroimdifigs that may influence it, including possible enemies, possible food, 

 possible competitors. 



We must not read into the story our own likes and dislikes. The wolf and 

 the vulture may be just 2isfit as the sheep and the chicken. The thistle and 

 the ragweed are just asfltas the fig-tree and the rose. But no plant species and 

 no animal species can altogether fit in where some other one is now living. 

 The "fitness" of a form, or its adaptation to its surroundings, is of a special 

 kind that it has taken hundreds of thousands of years to attain. When the 

 conditions in any region change radically, the character of the entire vegeta- 

 tion and of the animal life must also change. 



What All Species Need All protoplasm depends eventually upon 

 water and air, upon the same few chemical elements, and upon the same 

 classes of chemical compounds. Yet the countless forms of plant and animal 

 life find congenial surroundings in nearly all parts of the world, whereas each 

 species is closely restricted to a rather narrow range of temperature and 

 moisture. We have all been impressed by the striking differences between 

 tropical forms and related arctic forms, or between water animals and related 

 land animals (see illustration, p. 548 ). 



We are accustomed to expect polar bears in Greenland rather than in the 

 Everglades. In Florida we should expect to find alligators. The Canada lynx 

 is distributed throughout a large part of Canada and in some of the northern 



546 



