Thus _physiological psychology_ was raised by psychophysics to the rank of a physical and, in principle, exact science. But it had already obtained solid foundations in other provinces of biology. Comparative psychology had traced connectedly the long gradation from man to the higher animals, from these to the lower, and so on down to the very lowest. At the lowest stage it found those remarkable beings, invisible with the naked eye, that were discovered in stagnant water everywhere after the invention of the microscope (in the second half of the seventeenth century) and called "infusoria." They were first accurately described and classified by Gottfried Ehrenberg, the famous Berlin microscopist. In 1838 he published a large and beautiful work, illustrating on 64 folio pages the whole realm of microscopic life; and this is still the base of all studies of the protists. Ehrenberg was a very ardent and imaginative observer, and succeeded in communicating his zeal for the study of microscopic organisms to his pupils. I still recall with pleasure the stimulating excursions that I made fifty years ago (in the summer of 1854) with my teacher, Ehrenberg, and a few other pupils--including my student-friend, Ferdinand von Richthofen, the famous geographer--to the Zoological Gardens at Berlin. Equipped with fine nets and small glasses, we fished in the ponds of the Zoological Gardens and in the Spree, and caught thousands of invisible micro-organisms, which then richly rewarded our curiosity by the beautiful forms and mysterious movements they disclosed under the microscope. The way in which Ehrenberg explained to us the structure and the vital movements of his infusoria was very curious. Misled by the comparison of the real infusoria with the microscopic but highly organised rotifers, he had formed the idea that all animals are alike advanced in organisation, and had indicated this erroneous theory in the very title of his work: _The Infusoria as Perfect Organisms: a Glance at the Deeper Life of Organic Nature_. He thought he could detect in the simplest infusoria the same distinct organs as in the higher animals--stomach, heart, ovaries, kidneys, muscles, and nerves--and he interpreted their psychic life on the same peculiar principle of equally advanced organisation.