Such researches might unfold some unexpected laws in the succession of infusorial life. Those germs which are most widely diffused, will be the most likely to be developed in any mass of convenient water; but how and why the rarer forms come and go is very imperfectly understood. Slight modifications in surrounding circumstances will materially affect the result. Thus, if we bring home a handful of conferva, and a few water-plants of higher organisation, such as duckweed and anacharis, and place the whole in a glass jar full of pond-water, we shall at first have a good stock of objects; but they will usually grow less and less, until scarcely anything is left. If, however, we introduce a few pieces of straw, or a tiny wisp of hay, we shall succeed much better, and not only preserve our population longer, but enjoy a succession of animated crops. Extensive decomposition of vegetable matter kills off all but certain families, such as Paramecia, who enjoy it; on the other hand, too little decomposition proves fatal to some creatures, by depriving them of their food, and when they have died off, those who depended upon them for a living, die too. Different vegetables in decomposition suit different creatures, and hay and straw in that state seem to please the largest number. An animalcule tank will succeed best when it contains two or three kinds of growing plants, which oxygenize the air, and a moderate variety of decomposing organisms will supply food without making the water offensive. From these considerations it will be apparent that not only the nature of the vegetation of a pond, which is often changed by accidental circumstances, but also the quality of the odds and ends that the winds may blow into it, or which may fall through the air, will do much to determine the character and number of its inhabitants, while the quantity of shade or sunshine it enjoys, will also exercise an important influence. Hay and other infusions have from the beginning of microscopic investigations been employed to obtain the creatures which the Germans call "Infusions thierchen" (infusion animalcules), and the English "Infusoria;" but very little has yet been done in the way of their scientific culture and management.