14 INTRODUCTION TO EVOLUTION 



and mice that dwell with us in our houses as unwelcome guests originated 

 in Asia but are so adaptable to all manner of conditions that they have be- 

 come practically world-wide in distribution. Apparently cockroaches, 

 which have survived virtually unchanged all the vicissitudes of the earth 

 since the days before the great coal deposits were formed, owe their vast 

 lease on life to a similar ability to adapt to whatever may befall them. 

 Other examples might be cited but we may content ourselves by men- 

 tioning that man himself, thanks to employment of his intelligence, is the 

 supreme example of ability to live under widely diverse conditions. 



A third way in which animals may meet changes in environment is by 

 undergoing changes themselves. This is the commonest method of solving 

 the problem, by species that do succeed in solving it. In Chapter 15 we 

 shall discuss a theory that changes in the environment directly produce or 

 call forth appropriate corresponding changes in animals — the theory of the 

 inheritance of acquired characters. Since, as we shall see, there is little 

 positive evidence that this phenomenon does occur, we shall concentrate 

 attention here upon mutations as a source of bodily change in animals. 

 As we shall see later, there are other sources of genetic variability than 

 mutations. But in order to present the principles of natural selection as 

 simply as possible we shall concentrate on mutations in our present dis- 

 cussion. 



As noted before, mutations are changes in genes that result in changes 

 in the bodies of animals possessing the changed genes in appropriate pro- 

 portion (see pp. 331-335). Mutations occur at random, without regard 

 to the needs of individuals in which they occur. If the mutations are harm- 

 ful in their effect, they will be eliminated by the negative action of natu- 

 ral selection discussed above. If, however, the mutations are beneficial, 

 they will be preserved, and the number of individuals possessing them 

 will be increased in subsequent generations by the positive action of natu- 

 ral selection. Recent experiments showing that natural selection actually 

 does operate as we postulate are briefly discussed in Chapter 20. Posi- 

 tive selection of beneficial mutations may lead to more perfect adaptation 

 to an existing environment, or, alternatively, to adaptation to new con- 

 ditions if the environment changes. 



We have seen that when the environment changes a species may be- 

 come extinct. Sometimes, however, though most of the members of a spe- 

 cies may fail to survive, a few members, usually the possessors of changes 

 (mutations) adapting them to the new conditions, will survive. The sur- 

 viving few will then become the progenitors of future generations, which 

 will inherit the changed condition permitting life in the changed environ- 

 ment. 



