Now carbon dioxide is a product of the organic waste of living substance; it is given off by active Paramecia. Where therefore many are collected together they form a centre of the production of this substance; and when other Paramecia come, in the course of their random movements, into such a centre they remain there and help to swell the numbers in the cluster. If Paramecia be placed in water to which a distinctly reddish tinge is given by mixing it with a small quantity of rosol--a substance which is decolourized by carbon dioxide, and is not injurious to Paramecia--it will be seen that, where the groups are collected, the reddish tinge fades and disappears. As the groups expand, and are less densely packed, the colourless area expands too: and the limits within which the group is circumscribed are also the limits of decolourization. Dr. Jennings considers it beyond question that the assembling of Paramecia is due to the presence in such assemblages of carbonic acid produced by the animals themselves. The first beginning of the crowd may be some small fragment of bacterial clot or other substance. It would seem, then, that Paramecia are attracted by faintly acid solutions; and here at least there is, it may be urged, an element of choice. But even here, according to Dr. Jennings, there is not only no real choice, but not even any real attraction. What takes place, according to his observations, is briefly as follows. Suppose a faintly acid drop be inserted beneath the cover-glass. Paramecia may almost graze its boundary without being in any way affected by its presence. But in their random movements some, and eventually many, perhaps most, of the little animals chance to enter the faintly acid region; but there is no sign of reaction or response; they swim on across the drop until they reach its further margin. Here a reaction does take place. Instead of proceeding onwards, slowly revolving on its long axis, a Paramecium thus situated jerks backwards by a reversal of all the cilia, at the same time revolving on its axis in a direction opposite to that in which it was before turning. But the cilia of the mouth-groove resume their normal mode of working sooner than the others, and this causes the Paramecium to turn aside. It then goes ahead until it again reaches the boundary at another point, when the same behaviour is seen. The course of such a Paramecium is shown in Fig. 2, IV.