An account of the probable genesis of the memory images not only furnishes a clue to the mechanism of dreaming but to the underlying conditions as well. The lowest forms of life possess no image-forming power. They have no sense organs; sensation is diffused over the entire form and undifferentiated. Gradually, as the scale of life is ascended, certain parts of the organisms become specially sensitive to certain stimuli and eventually individual organs give separate and distinct reports of phenomena. A substance hitherto merely felt, is seen, heard, smelled, tasted. The passage from sensation to perception occurs when but one or two of the sense organs are stimulated by an object, yet, because of nervous connections established during former more close and complete experience of the object the remaining sense organs are faintly roused, sending into consciousness copies of former sensations. Thus the whole is present to mind while but a part to sense. In the developing brain the store of memory images of various kinds would rapidly increase and these images would come at length to have a more or less independent existence. It is probable that the next step in the making of mind was the synthesis of one set of sense impressions to form an idea of the object, the first abstraction, and thenceforth a sensation gave rise to an idea. There is at this stage no impulse to explain sensations, but involuntarily, from the store of memory images, and from the reservoir of ideas above, emerges a representation of the exciting object. If this is one to which the organism is accustomed the resulting complex in the highest nerve centers fits the subject, but as evolution proceeds and environment and capacity for sensation grow more complex, new stimulations occur. In the absence of the capacity for knowledge and understanding of the object the developing mind, true to its law, brings forward mental images most nearly related--those which fit in one or two respects,--and thus we have the birth of analogy, "the inference of a further degree of resemblance from an observed degree of resemblance."