All organisms, animal as well as vegetable, are at some period of their existence provided with an opportunity of migration. In the animal world, most land creatures have legs or wings, which allow them to roam about freely--a freedom which is of special importance as enabling them to obtain nourishment and to avoid disadvantageous conditions. Aquatic animals are likewise to a great extent possessed of powers of locomotion, but such powers are not so essential to them as to terrestrial creatures, since the water itself is full of small organisms, both animal and vegetable, on which they can feed; hence a large variety of water creatures are content to remain during much of their lives fixed to one spot, extracting from the water as it passes by both the supply of organic food and the inorganic substances, such as oxygen or carbonate of lime, which they require for their life processes. These sedentary creatures, of which barnacles, sea-anemones, and zoophytes will serve as examples, once attached, do not move from the spot where they have settled down; but it is important to note that not only are their eggs or young mostly liberated into the water, and by it transported to new homes, but in their juvenile stages they often swim vigorously, and thus achieve a wide dispersal. In the plant world, the higher forms, with very few exceptions, spend their lives attached to one spot, like sea-anemones, deriving their food-supply from the air and from the soil; but they similarly are given the opportunity, after birth, of migrating. In our familiar wild flowers, for instance, the young plant, at an early stage of its existence, while it is still minute, becomes covered with a coat often of very resistant qualities, and is then cast loose by the parent in the form of _seed_, mostly in great numbers, to achieve what travels it can before it takes root and settles down, like its parent before it, to a humdrum existence. In the Cryptogams, or so-called Flowerless Plants, this temporary compression of the organism into very narrow limits suitable for easy dispersal takes place at a different period in the life cycle, but for mechanical purposes the results are similar. Minute bodies, or _spores_ (much smaller than the seeds of the Seed Plants), are cast loose by the parent often in vast numbers, and eventually settle down and reproduce the species. In many of the lower aquatic plants these spores are provided with means of locomotion in the form of a tail-like appendage, which by its movement propels the germs through the water, giving them the same advantage which is possessed by the young of many of the sedentary animals.