Now by frequent repetition the surprise, incongruity, or novelty ceases; and, in consequence, the pleasure or pain which accompanied it, and also the degree of volition which was excited by that sensation of pain or pleasure; and thus the sensorial power of sensation and of volition are subducted from the catenation of vital actions, and they are in consequence produced much weaker, and at length cease entirely. Whence we learn why contagious matters induce their effects on the circulation but once; and why, in process of time, the vital movements are performed with less energy, and at length cease; whence the debilities of age, and consequent death.
ADDITIONAL NOTES. VIII.
REPRODUCTION.
But Reproduction with ethereal fires
New life rekindles, ere the first expires.
Canto II. l. 13.
I. The reproduction or generation of living organized bodies, is the great criterion or characteristic which distinguishes animation from mechanism. Fluids may circulate in hydraulic machines, or simply move in them, as mercury in the barometer or thermometer, but the power of producing an embryon which shall gradually acquire similitude to its parent, distinguishes artificial from natural organization.
The reproduction of plants and animals appears to be of two kinds, solitary and sexual; the former occurs in the formation of the buds of trees, and the bulbs of tulips; which for several successions generate other buds, and other bulbs, nearly similar to the parent, but constantly approaching to greater perfection, so as finally to produce sexual organs, or flowers, and consequent seeds.
The same occurs in some inferior kinds of animals; as the aphises in the spring and summer are viviparous for eight or nine generations, which successively produce living descendants without sexual intercourse, and are themselves, I suppose, without sex; at length in the autumn they propagate males and females, which copulate and lay eggs, which lie dormant during the winter, and are hatched by the vernal sun; while the truffle, and perhaps mushrooms amongst vegetables, and the polypus and tænia amongst insects, perpetually propagate themselves by solitary reproduction, and have not yet acquired male and female organs.
Philosophers have thought these viviparous aphides, and the tænia, and volvox, to be females; and have supposed them to have been impregnated long before their nativity within each other; so the tænia and volvox still continue to produce their offspring without sexual intercourse. One extremity of the tænia, is said by Linneus to grow old, whilst at the other end new ones are generated proceeding to infinity like the roots of grass. The volvox globator is transparent, and carries within itself children and grandchildren to the fifth generation like the aphides; so that the tænia produces children and grandchildren longitudinally in a chain-like series, and the volvox propagates an offspring included within itself to the fifth generation; Syst. Nat.
Many microscopic animals, and some larger ones, as the hydra or polypus, are propagated by splitting or dividing; and some still larger animals, as oysters, and perhaps eels, have not yet acquired sexual organs, but produce a paternal progeny, which requires no mother to supply it with a nidus, or with nutriment and oxygenation; and, therefore, very accurately resemble the production of the buds of trees, and the wires of some herbaceous plants, as of knot-grass and of strawberries, and the bulbs of other plants, as of onions and potatoes; which is further treated of in Phytologia, Sect. VII.
The manner in which I suspect the solitary reproduction of the buds of trees to be effected, may also be applied to the solitary generation of the insects mentioned above, and probably of many others, perhaps of all the microscopic ones. It should be previously observed, that many insects are hermaphrodite, possessing both male and female organs of reproduction, as shell-snails and dew-worms; but that these are seen reciprocally to copulate with each other, and are believed not to be able to impregnate themselves; which belongs, therefore, to sexual generation, and not to the solitary reproduction of which I am now speaking.