A fair result can be obtained only by fully stating and balancing the facts and arguments on both sides of each question; and this is here impossible.

I much regret that want of space prevents my having the satisfaction of acknowledging the generous assistance which I have received from very many naturalists, some of them personally unknown to me.

I cannot, however, let this opportunity pass without expressing my deep obligations to Dr.

Hooker, who, for the last fifteen years, has aided me in every possible way by his large stores of knowledge and his excellent judgment.