In the author''s studies on reproduction and sex these functions have been shown to be regulated, to a hitherto unrecognized extent, by the organs of internal secretion. In a provisional classification of true hormones it is observed that they are all identified with the regulation of rhythmic processes; particularly with highly irregular rhythms[long dash]including the species-preserving rhythms of reproduction. The true hormones arose late in animal life, and nearly all of those now known are confined to vertebrates. A consideration of the evolutional origin of the coordinating mechanisms of the body leads to the view that the nervous system, probably both central and sympathetic, arose earlier than the true hormones. Starling''s widely accepted views thus require restatement. First came the simple forms (non-hormonal) of chemical regulation; next, neural regulation; last, true hormonal regulation.