IN relation to your reviewer's interesting notice in NATURE for June 27 of Prof. Wood-Jones's booklet, “The Problem of Man's Ancestry,” it is appropriate to remember that the “blood-reaction test” shows the relationship of man to the ape to stand exactly as that of the horse to the donkey; the latter have had a common ancestor. Taking man as equalling 100, the ape comes at 70; the numbers for the horse and the donkey are the same. But this test shows no blood relationship whatever between man and the lower animals, thus confirming Klaatsch's view that the human line became separated very far down at the basis of the vertebrate phylum.