I did this the more willingly because the limited time at my disposal for any studies of the kind seemed more likely to suffice for a monograph upon a single and definite problem than for an investigation into larger questions. It soon became manifest, however, that to elaborate all the points of interest that offered themselves at almost every step of even my more limited way would tax the powers of an inquirer who could devote his whole time to the work. Hence, although I have found myself able to consider a few of the more important of such points in separate Appendices, yet as regards many others I have been compelled to rest content with mere hints or bare statements, in the hope of resuming the study of them at a future time.