In the previous chapter the idea is sought to be conveyed that we originate nothing in our dreams. We may conceive of things which never existed, or of which we have heard or read, but the images we make of them are either composed of elements familiar to us, or else are based upon ideal representations which we have formed in our waking moments. Thus, before the discovery of America no Europeans ever dreamed of American Indians, for the reason that nothing existed within their knowledge which could give any idea of the appearance of such human beings. It is possible that Columbus and his companions may have dreamed of the continent of which they were in search and of its natives, but the images formed of the latter must necessarily have resembled other beings they had seen, or which they had heard described. After the discovery, however, it was no unusual thing for the Spaniards and others to have correct images of Indians appear to them in their dreams. Dreams, therefore, must have a foundation, and this is either impressions made upon the mind at some previous period, or produced during sleep by bodily sensations. These impressions, however they may be formed, are subjected to the unrestrained influence of the imagination. At first sight it may seem that we often have dreams not excited by actual sensations, and which have no relation to any events of our lives, or any ideas which have passed through our minds, but thorough investigation will invariably reveal the existence of an association between the dream and some such ideas or events. For instance, a few nights ago I dreamed that a gentleman, a friend of mine, had invented what he called a “dog-cart ambulance,” a vehicle which he declared was the best ever made for the transportation of sick or wounded men. On awaking, all the particulars were fresh in my mind, but I could not for some time perceive why I had had such a dream. At last I recollected that the morning before a gentleman had given me a very full description of Prospect Park, in Brooklyn. The friend of whom I dreamed has charge of the construction of this Park. His presence was, therefore, fully explained, and as dog-carts are driven in parks, this link was also accounted for. The ambulance part was due to the fact that I had that same morning found the card of a gentleman upon my table who really had invented an ambulance. The imagination had, therefore, taken these data supplied by the memory, and had combined them into the incongruous web constituting my dream. Dreams are also frequently built upon circumstances which have transpired many years previously, and which have long since apparently passed from our recollection. A very striking instance of this kind is related by Abercrombie,[72] on the authority of Sir Walter Scott.