A PHENOMENON not rare in the evolution of science is that professional men of erudition, with all the help at their disposal, with all the implements of their knowledge and ability, combat some principle of popular wisdom which is, on the other hand, defended by the people with equal tenacity, and that finally science is forced to recognise that in essentials the popular conception, and not its own, is the correct one. It would be especially worthy of investigation to discovery why it is that science, on its gradually mounting path, progresses in an irregular zigzag line, which at times comes close to the popular view of the world, and at times quite departs from it.

I mention this peculiar phenomenon for the reason that the latest investigations of dreams, those remarkable and bizarre manifestations of mental life,