The nervous system in its relative degree of development and complexity becomes the ultimate standard by which the perfection of animals is estimated, and their place in the scale of creation assigned to them : if man stand alone and unattended, as he undoubtedly does, upon the summit of the pyramid, it is only because he possesses in his brain the organs of certain moral and intellectual facul ties which occur in no other living thing; these confer on him his humanity; these are the ma terial parts to which the soul is wedded during his existence.
In intimate connection with the functions of phrenic or animal life, and developed nearly in the same ratio, is the muscular system, the most universal agent of locomotion. Exceed ingly simple at first, and operating at great disadvantage through a want of levers and points of support, we trace it becoming gra dually more complicated as we ascend, and, finally, provided with a complementary skeleton or frame-work by means of which it acts to the best advantage. The skeleton among animals is of two kinds,—external and horny, internal and osseous. In the first case the muscular system is inclosed within the resisting pieces which it has to move; in the second it is without these, and is arranged around them. The bones and muscles together compose the numerous and variously fashioned instruments with which animals accomplish the promptings of their inward appetites and instincts. They form feet, fins, hands, the prehensile tail, &c. The muscular system, and a modification of the osseous, the cartilaginous, moreover, com pose the most universal instrument by which animals communicate their vicinity, their states, their dispositions or affections, &c. to one ano ther—this is the larynx.
The means by which species are continued, are extremely varied. The very lowest tribes of animals we have seen shooting forth buds exactly like vegetables, and these being in due season detached from the body of the parent, find themselves fitted to commence an inde pendent existence. At the next step we take in ascent, however, we meet with particular organs of reproduction ; and, singular enough, the moment these exist they are not of one, but of two kinds, denominated male and fe male. Sometimes these organs are possessed by single individuals, far more commonly, however, they are divided between two, whence the so uniform division of the beings com posing the animal kingdom into sexes. The
simplest form of the male organ of generation is a gland secreting a fecundating fluid (the testis) and an excretory duct : the simplest form of the female apparatus of generation is a gland or body producing germs (the ovary) and an excretory duct. In a greater state of complication or development these essential parts in the male have an instrument super added to them by which the fecundating fluid is carried directly into the body of the female, and in the female the ovary has a dilatable cavity superadded in which the germ remains for a season, and until its included embryo attains such a state of development as is com patible with its more independent existence surrounded by the circumstances amid which it is afterwards to live. In the higher classes, the connection between the parent and offspring does not cease immediately on the birth of the latter, and in the highest of all we find the female furnished with a complementary apparatus (the from which she fur nishes her young with food during the first period of its existence.
Actions of animals.—The foregoing rapid sketch of the grand features of distinction among animals with reference to their struc ture naturally leads to the inference of di versity of function in harmony with the pecu liar organization possessed by each. In the lowest grades of animal existence we have seen to how simple a process the act of nutrition —this act so complicated among the more elevated tribes,—is reduced. It consists merely of imbibition or absorption by and of exha lation from the general surface of the body. The matters absorbed appear to be assimilated incontinently, or to be made a part of, and to receive the form proper to, the animal in the instant of their assumption : applied imme diately to the homogeneous organism, the nutriment is forthwith made a portion of its substance. The vital decomposition of the bodies of these lower animals is accomplished with the same simplicity and directness : the surface that absorbs is also that which exhales the worn-out particles of the system.