54 THE EVOLUTION THEORY 



the stem and leaves of some of the individuals, as may easily happen 

 through very slight changes in the chemical metabolism of the plant, 

 what, then, could result but that such individuals would be less 

 readily eaten than the others ? A process of selection must, therefore, 

 ensue, and the unpleasant-tasting specimens of the plant would be 

 much more frequently spared, and consequently would bear seed 

 much oftener than the palatable ones. Thus the number of un- 

 palatable plants would increase from year to year. If the stuff in 

 question were not only unpalatable but poisonous, or gradually 

 became so, a plant would in time be evolved which would be 

 absolutely safe from being devoured by animals, just as the deadly 

 nightshade {Atropa belladonna) actually is. 



Or let us suppose that a stretch of water is inhabited by a species 

 of carp, which have hitherto had no large enemy, and so have become 

 lazy and slow, and that there migrates from the sea into this stretch 

 of water a large species of pike. At first numerous carp will fall 

 victims to the pike, and the pike will rapidly increase in number. 

 But if all the carp were not equally lazy and dull-witted, if some of 

 them were quicker and more intelligent, these would, on an average, 

 become more rarely the victims of the pike, and numerous individuals 

 with these better qualities would survive in each generation, till 

 ultimately there were no others, and the useful characters would 

 gradually become intensified, and so a more active and wary race of 

 carp would arise. 



Let us suppose, however, that the increased activity and 

 wariness would not alone suffice to preserve the colony from ex- 

 tinction ; it might require also an increased fertility to prevent the 

 normal number from being permanently lowered ; but even this could 

 eventually be brought about by natural selection, if the nature of 

 the species and the general conditions of its life permitted. For there 

 are variations of fertility in every species, and if the chance of seeing 

 some of its eggs become mature animals were greater for the more 

 fertile female than for the less fertile, ceteris paribus, a process 

 of selection must take place, which would result in an increase of 

 fertility as far as that was possible. 



Obviously, such processes of natural selection can affect all parts 

 and characters— size and form of the body, as well as isolated 

 parts, the external skin and its colour, every internal organ — and 

 not bodily characters alone, but psychical ones as well, such as 

 intelligence and instincts. According to this principle, it is only 

 characters which are biologically indifferent that cannot be altered 

 through natural selection. 



