Bacteria, which are generally looked upon as unicellular plants, appear, despite the earlier statements to the contrary, to react in much the same way as do the protozoans, according to the recent work of Rothert, and of Jennings and Crosby. The bacteria do not seem to turn toward or away from chemical substances, but they collect in regions containing certain substances in much the same way as do the protozoans. The collecting of bacteria in regions where oxygen is present has been known for some time, but it appears from more recent results that they are not attracted toward the oxygen, but by accidentally swimming into a region containing more oxygen they are held there in the same way as is paramœcium in a drop of acid. On the other hand bacteria do not enter a drop of salt solution, or of acids, or of alkalies. They react negatively to all such substances. Some kinds of bacteria have a flagellum at each end, and swim indifferently in either direction. If they meet with something that stimulates them, as they move forward, they swim away in the opposite direction, and continue to move in the new direction until something causes again a reversal of their movement. In this respect their mode of reaction seems of greater advantage than that followed by paramœcium. Another instinct, that appears to be due to a tropic response, is the definite time of day at which some marine animals deposit their eggs. The primitive fish, Amphioxus, sets free its eggs and sperm only in the late afternoon. A jellyfish, Gonionema, also lays its eggs as the light begins to grow less in the late afternoon, and in this case it has been found that the process can be hastened if the animals are placed in the dark some hours before their regular time of laying. There is no evidence that this habit is of any advantage to the animal. We may imagine, if we like, that the early stages may meet with less risk at night, but this is not probable, for it is at this time that countless marine organisms come to the surface, and it would seem that the chance of the eggs being destroyed would then be much greater. It is more probable that the response is of no immediate advantage to the animals that exhibit it, although in particular cases it may happen to be so.