Now in the definition on which, as the representative of a whole class, we are _now_ animadverting, a single effect is given as constituting the cause. For nutrition by digestion is certainly necessary to life, only under certain circumstances, but that life is previously necessary to digestion is absolutely certain under all circumstances. Besides, what other phenomenon of Life would the conception of assimilation, _per se_, or as it exists in the lowest order of animals, involve or explain? How, for instance, does it include sensation, locomotion, or habit? or if the two former should be taken as distinct from life, _toto genere_, and supervenient to it, we then ask what conception is given of _vital_ assimilation as contradistinguished from that of the nucleus of a crystal? _Lastly_, this definition confounds the Law of Life, or the primary and universal form of vital agency, with the conception, Animals. For the kind, it substitutes the representative of its degrees and modifications. But the first and most important office of science, physical or physiological, is to contemplate the power in kind, abstracted from the degree. The ideas of caloric, whether as substance or property, and the conceptions of latent heat, the heat in ice, &c., that excite the wonder or the laughter of the vulgar, though susceptible of the most important practical applications, are the result of this abstraction; while the only purpose to which a definition like the preceding could become subservient, would be in supplying a nomenclature with the character of the most common species of a genus—its _genus generalissimum_, and even this would be useless in the present instance, inasmuch as it presupposes the knowledge of the things characterised.