In the animal kingdom the entire adult organism, as well as each of its separate parts, has certain dimensions, beyond which, under ordinary circumstances, it does not pass, either in the one direction or the other. It may not be easy or possible to state what the limits are, but, practically, this inability to frame a precise limitation is productive of no inconvenience. It is universally admitted that a certain animal attains such and such dimensions, and that one organ has a certain proportionate size as contrasted with another. The same rules hold good in the case of plants, though in them it is vastly more difficult to ascertain what may be called the normal dimensions or proportions. Nevertheless observation and experience soon show what may be termed the average size of each plant, and any disproportion between the several organs is speedily detected. When there is a general reduction in size throughout all the organs of a plant, or throughout all the nutritive organs, stem, leaves, &c., and the several portions participate in this diminished size, we have what are generally termed "dwarf varieties," dwarf in comparison, that is, with the ordinary condition of the plants; on the other hand, if the entire plant, or, at least, if the whole of one set of organs be increased in size beyond the recognised average, we have large varieties, often qualified by such terms as _macrophylla_, _longifolia_, _macrantha_, &c. &c. In all these cases either the entire plant or whole series of organs are alike increased or diminished beyond average limits; and such variations are often very constant, and are transmitted by hereditary transmission. It may be supposed that such deviations may have originated, in the first instance, either from excessive use, or from disuse, or from the agency of certain conditions promoting or checking growth, as the case may be; but whether or no, it is certain that these variations often persist under different conditions, and that they often retain their distinctive characters side by side with plants presenting the normal average dimensions. In other cases the variations in size are of a less general character, and affect certain organs of a whorl in a relative manner, as, for instance, in the case of didynamous or tetradynamous stamens, where two or four stamens are longer than their fellows, the long or short stamens and styles of di- and tri-morphic flowers, &c. These differences are sometimes connected with the development of parts in succession, and not simultaneously. Teratological deviations of size differ from those of which mention has just been made chiefly in this, that they are more limited in their manifestations. It is not, as a rule, the whole plant, or the whole series of nutritive or of reproductive organs, that are affected, but it is certain parts only; the alteration in size is more a relative change than an absolute one.