A few illustrations of this reciprocal action must be given.
_Illustrative Cases of the Struggle for Life_.
Sir Charles Lyell observes that if, by the attacks of seals or other marine foes, salmon are reduced in numbers, the consequence will be that otters, living far inland, will be deprived of food and will then destroy many young birds or quadrupeds, so that the increase of a marine animal may cause the destruction of many land animals hundreds of miles away.

Mr.Darwin carefully observed the effects produced by planting a few hundred acres of Scotch fir, in Staffordshire, on part of a very extensive heath which had never been cultivated.

After the planted portion was about twenty-five years old he observed that the change in the native vegetation was greater than is often seen in passing from one quite different soil to another.

Besides a great change in the proportional numbers of the native heath-plants, twelve species which could not be found on the heath flourished in the plantations.

The effect on the insect life must have been still greater, for six insectivorous birds which were very common in the plantations were not to be seen on the heath, which was, however, frequented by two or three different species of insectivorous birds.