In the preface of this book, he writes: "...the chief aim of the work is to present in a popular and readily understood form, the chief evidences of the evolution of living beings. In this view, whilst I have been content to assume the reality of that process, I have also endeavoured to marshal the more prominent facts of zoology and botany, which serve to prove that evolution, broadly considered is not merely a name for an unknown tendency in nature, but is an actual factor in the work of moulding the life with which the universe teems."