Or let us think of a herbaceous plant, in appearance something like a belladonna, rich in leaves and very juicy, but not poisonous. It would doubtless be a favourite food with the animals of the forest, and it would not, therefore, attain to more than a sparse occurrence, since few of the individuals would be able to form seeds. But now let us assume that a stuff of very unpleasant taste develops in 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 unpalatable 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 extinction; 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.