Originally published in 1932, this book examines how hormones and nervous impulses affect the body, with special reference to animals with colour-changing abilities. Parker gives examples from various areas of the animal kingdom, both vertebrates and invertebrates, to demonstrate how the stimulation of certain sensory organs and nerves can produce very different effects and to draw wider conclusions about the role that 'nervous secretion' can play in other physiological operations. This book will be of value to anyone with an interest in the history of science and comparative physiology.