Every plant which grows on the surface of the earth or in the waters constitutes a distinct individuality. The careful examination and comparison of a certain number of these individuals of the vegetable world will lead to the admission that a great many are quite identical in some of their characteristics, while others possess no character in common. Examine the individual plants, for instance, which compose a field of oats; in each the root, the stem, the flowers, the fruit, present the same identical characters. The seed of any one whatever of these plants will yield other plants like those of the field. Every individual in the field belongs therefore to the same _species_--to the species Avena sativa. The species, then, is a collection of all the individuals which resemble each other, and which will reproduce other individuals like themselves. These species may present, as the result of diverse influences, such as change of climate or cultivation, differences more or less marked, more or less persistent, which withdraw them from the original type. To these, according to their importance, botanists give the name of _varieties_ and _sub-varieties_. The wheat-plant, the vine, the pear, the apple, and most of our cultivated legumes, all yield, under the influence of culture extending over a long series of years, plants altogether different from the original in their exterior; but they preserve, one and all, the essential characters of the species. They are _varieties_ of the wheat-plant, of the vine, of the pear, of the apple. The assemblage of a certain number of distinct species presenting the same general characteristics, the same disposition of organs, the same structure of flower and fruit, constitutes a group to which the name of _genus_ is applied. Rosa canina, R. villosa, and R. Sabini are three different species of the same group--the genus Rosa. The words _oak_, _poplar_, _barley_, are collective common names, which served, long before botanical science existed, to designate certain groups of plants. These are true generic names of popular creation, which botanists have accepted because they were the result of exact observation. “A man of observant eye and quick intelligence,” says Auguste Pyramus de Candolle, “would observe certain groups in the vegetable kingdom which we call genera before discerning the species.”