Alfred Russel Wallace (1823-1913) was the co-creator of the theory of Natural Selection, and his pending publication of his ideas prompted Darwin to finally publish his own book in 1859. While Wallace himself never wrote a "major" theoretical book, this 1889 book is an interesting statement of his mature ideas on the subject.
He notes that Darwin "did his work so well that descent with modification is now universally accepted as the order of nature in the organic world." "The idea of special creation or any altogether exceptional mode of production is absolutely extinct!"
Wallace rejects several portrayals of Evolutionary theory. For example, he rejects the theory of the inheritance of acquired characteristics (which Darwin accepted), "there is much reason for believing that such acquired characters are in their nature non-heritable." He also writes, "the poet's picture of 'Nature red in tooth and claw'... is a picture of evil which is read into it by our imaginations."
His later advocacy of Spiritualism (e.g., Miracles and Modern Spiritualism, A Defence Of Modern Spiritualism, etc.) discredited Wallace in the eyes of some scientifically-minded persons. Nevertheless, he concludes the book on the note, "the Darwinian theory ... lends a decided support to, a belief in the spiritual nature of man."
This is an often-overlooked, but very illuminating book for anyone intersted in the development of evolutionary theory.