Assimilation appears to be identical in all animals ; it is the ultimate term of nutrition, and however varied the apparatus that minis ters to the act, the act itself we may presume not to differ in its essence in one animal from what it is in another.
Akin to assimilation we have secretion, and this is a function that offers extensive differences in every class of the animal kingdom. It is generally spoken of as of two kinds, excretion, and secretion, properly so called. In the lowest tribes excretion is quite simple, consisting of a mere exhalation from the general surface of the body. In the more elevated we find another and very important form of excretion super added, that, namely, of the urine, the nature of which, and the mode in which it takes place, we have already indicated in speaking of the structure. Secretion, however, even in .the classes but a little raised above the lowest, is a function of much more varied import, and con sists of a great many other processes than that by which the bodies of animals are depurated and their blood maintained in a state fit to supply all the wants of the system. We ad vance but a little way before we begin to detect distinct organs destined for the secretion of peculiar fluids from the general mass of cir culating nutriment, evidently subservient in many cases to the most important ends of the economy, and by no means destined to be rejected from the system as useless, like the excretions properly so called. It seems even that it is by a process analogous to secretion that the imponderable matters—the heat, light, and electricity, which we have acknowledged as elements in the constitution of organized beings, are eliminated.
All animals possess sensibility or sensation, though evidently in the most dissimilar degrees. Some have been supposed to possess the faculty of perceiving impressions made upon them by external objects, but to have no power of re acting upon external nature, they being without the faculties which in the higher classes prompt to action. This however, of animal ex istence is rather hypothetical than demonstrable, and in animals generally we observe not only the aptitude to be impressed, but inherent capacities inducing reaction upon the world around them. The sensitive life of these beings consequently consists of two items—the senses and their organs, external and internal, by which im pressions are received and cognized, and the affective and intellectual faculties by which the motives to action, the propensities, sentiments, instincts, appetites, &c., are originated, and
the means and modes of accomplishing their promptings are supplied.
Animals evidently differ immensely in the degrees in which they are endowed with ex ternal and internal senses. Some appear to possess none the external senses save touch ; others, in addition to this, have taste and smell ; the most perfect besides these three reckon sight and hearing. The internal senses, in like manner, are more or less acute, more or less numerous, according to the consitution of ani mals : those of hunger and thirst are probably universally distributed, and the most keenly felt; then come those which induce the respira-. tory act, the sexual act, &c. ; and here we find ourselves among the propensities which exist in very different numbers and kinds in every different species of animal. Some tribes tend their offspring, others leave their progeny to the care of accident, which in this case always suffices for their protection ; some con gregate in herds or shoals, others live solitary or in pairs ; some are bold and rapacious, others timid and gentle, &c. When we ex amine animals generally, with reference to the sentiments or moral faculties, we find them still more or less like each other in many respects, some being cautious or cowardly, proud or haughty, persevering or obstinate, &c., in various proportions. When we contrast all other animals with man, however, in regard to moral endowment, we immediately perceive the broad, the impassable line of difference that runs between the lord of creation and all the other beings that with him partake of life. The feeling which leads man to view his actions in their bearing upon others or in relation to jus tice, is extremely weak among animals, if in deed it do actually exist among them at all. The same may be said of the sentiment which leads mankind to wish well to all, and to succour and relieve those that are suffering and unfortunate. The feeling, again, that raises man to the imagination of a something beyond nature, the sentiment that inclines him to reve rence and adore his Maker, thus in one way re vealed to him, and the wonderful impulse that leads him to look beyond time and his merely temporary existence, and thence to conceive in finity and eternity, are so many moral attributes which man alone, of all created things, possesses.