In the first place it is illustrated in _structure_. The advance from the homogeneous to the heterogeneous itself involves an increasing distinction from the inorganic world. In the lowest _Protozoa_, as some of the Rhizopods, we have a homogeneity approaching to that of air, water, or earth; and the ascent to organisms of greater and greater complexity of structure, is an ascent to organisms which are in that respect more strongly contrasted with the relatively structureless masses in the environment. In _form_ again we see the same truth. A general characteristic of inorganic matter is its indefiniteness of form, and this is also a characteristic of the lower organisms, as compared with the higher. Speaking generally, plants are less definite than animals, both in shape and size--admit of greater modifications from variations of position and nutrition. Among animals, the _Amoeba_ and its allies are not only almost structureless, but are amorphous; and the irregular form is constantly changing. Of the organisms resulting from the aggregation of amoeba-like creatures, we find that while some assume a certain definiteness of form, in their compound shells at least, others, as the Sponges, are irregular. In the Zoophytes and in the _Polyzoa_, we see compound organisms, most of which have modes of growth not more determinate than those of plants. But among the higher animals, we find not only that the mature shape of each species is quite definite, but that the individuals of each species differ very little in size. A parallel increase of contrast is seen in _chemical composition_. With but few exceptions, and those only partial ones, the lowest animal and vegetal forms are inhabitants of the water; and water is almost their sole constituent. Dessicated _Protophyta_ and _Protozoa_ shrink into mere dust; and among the acalephes we find but a few grains of solid matter to a pound of water. The higher aquatic plants, in common with the higher aquatic animals, possessing as they do much greater tenacity of substance, also contain a greater proportion of the organic elements; and so are chemically more unlike their medium. And when we pass to the superior classes of organisms--land plants and land animals--we find that, chemically considered, they have little in common either with the earth on which they stand or the air which surrounds them.