Fitness in the Living World 3 2 7 



hydroids may be cut up into minute fragments and each / 

 piece will give rise to a typical animal. Some eggs may be 

 broken apart in the two-cell or four-cell stages and each cell 

 will give rise to a whole individual. Many lower animals 

 such as newts, crayfish, and worms have this power to a very 

 marked degree. The legs of a newt or crayfish may be cut 

 off again and again and yet maybe replaced after each ampu- 

 tation. In the regeneration of the legs of crabs, Morgan has 

 shown that those legs which are least liable to injury regen- 

 erate as readily as those which are most liable to be lost. If 

 the lens in the eye of a newt is removed, it will regenerate 

 more or less perfectly. Such individual adaptations cannot 

 be explained as the result of the inherited experiences of 

 former generations, since the injuries are frequently of such 

 a kind that they could never have occurred in nature. 



Higher animals do not have such extensive power of re- 

 generation, but every living thing has this power to a cer- 

 tain extent. Human beings cannot regenerate limbs or other 

 complex parts, but they have the power of healing wounds 

 and making repairs, otherwise cuts and other little injuries 

 would prove fatal. 



These individual adaptations are only samples of innum- 

 erable others that could be cited; indeed, individual adapta- 

 tions are almost if not quite as numerous as racial ones, and 

 they are even more mysterious and wonderful, since nothing 

 in the world seems more inexplicable than the ability of an 

 organism to respond in a useful and apparently purposive 

 way to conditions which it has never experienced before and 

 which in some instances even its ancestors could never have 

 experienced in all their past history. 



