The author's general aim has been to survey as wide a field of evidence as possible and this had involved excursions into subjects of which he has little first hand knowledge. This width of range also has necessitated a somewhat arbitrary selection of evidence and has prevented full discussion of any indi- vidual problem. The author trusts that he has not misrepresented anyone's results or opinions, and if this has occurred, he can only plead in excuse the peculiar difficulty of giving a brief and yet accurate account of evidence of such a wide variety. The diagrams reproduced in the article...