"The ingenious Dr. Hartley, in his work on man, and some other philosophers have been of opinion, that our immortal part acquires during this life certain habits of action or of sentiment which become for ever indissoluble, continuing after death in a future state of existence; and add that if these habits are of the malevolent kind, they must render their possessor miserable even in Heaven. I would apply this ingenious idea to the generation or production of the embryon or new animal, which partakes so much of the form and propensities of its parent.
"Owing to the imperfection of language the offspring is termed a new animal, but is in truth a branch or elongation of the parent, since a part of the embryon-animal is, or was, a part of the parent, and therefore in strict language, cannot be said to be entirely new at the time of its production; and, therefore, it may retain some of the habits of the parent system.
"At the earliest period of its existence the embryon would seem to consist of a living filament with certain capabilities of irritation, sensation, volition, and association, and also with some acquired habits or propensities peculiar to the parents; the former of these are in common with other animals; the latter seem to distinguish or produce the kind of animal, whether man or quadruped, with the similarity of feature or form to the parent."[169]
Going on to describe the gradual development of the embryo, Dr. Darwin continues:—
"As the want of this oxygenation of the blood is perpetual (as appears from the incessant necessity of breathing by lungs or gills), the vessels become extended by the efforts of pain or desire to seek this necessary object of oxygenation, and to remove the disagreeable sensations which this want occasions."[170]
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"The lateral production of plants by wires, while each new plant is thus chained to its parent, and continues to put forth another and another as the wire creeps onward on the ground, is exactly resembled by the tape-worm or tænia, so often found in the bowels, stretching itself in a chain quite from the stomach to the rectum. Linnæus asserts 'that it grows old at one extremity, while it continues to generate younger ones at the other, proceeding ad infinitum like a sort of grass; the separate joints are called gourd worms, and propagate new joints like the parent without end, each joint being furnished with its proper mouth and organs of digestion.'"[171]
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