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"The embryon" must "be supposed to be a living filament, which acquires or makes new parts, with new irritabilities as it advances in its growth."[176]
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"From this account of reproduction it appears that all animals have a similar origin, viz. a single living filament; and that the difference of their forms and qualities has arisen only from the different irritabilities and sensibilities, or voluntarities, or associabilities, of this original living filament, and perhaps in some degree from the different forms of the particles of the fluids by which it has at first been stimulated into activity."[177]
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"All animals, therefore, I contend, have a similar cause of their organization, originating from a single living filament, endued with different kinds of irritabilities and sensibilities, or of animal appetencies, which exist in every gland, and in every moving organ of the body, and are as essential to living organism as chemical affinities are to certain combinations of inanimate matter.
"If I might be indulged to make a simile in a philosophical work, I should say that the animal appetencies are not only perhaps less numerous originally than the chemical affinities, but that, like these latter, they change with every fresh combination; thus vital air and azote, when combined, produce nitrous acid, which now acquires the property of dissolving silver; so that with every new additional part to the embryon, as of the throat or lungs, I suppose a new animal appetency to be produced."[178]
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Here, again, it should be insisted on that neither can the "additional part" precede "the appetency," nor the appetency precede the additional part for long together—the two advance nearly pari passu; sometimes the power a little ahead of the desire, stimulates the desire to an activity it would not otherwise have known; as those who have more money than they once had, feel new wants which they would not have known if they had not obtained the power to gratify them; sometimes, on the other hand, the desire is a little more active than the power, and pulls the power up to itself by means of the effort made to gratify the desire—as those who want a little more of this or that than they have money to pay for, will try all manner of shifts to earn the additional money they want, unless it is so much in excess of their present means that they give up the endeavour as hopeless; but whichever gets ahead, immediately sets to work to pull the other level with it, the getting ahead either of power or desire being exclusively the work of external agencies, while the coming up level of the other is due to agencies that are incorporate with the organism itself. Thus an unusually abundant supply of food, due to causes entirely beyond the control of the individual, is an external agency; it will immediately set power a little ahead of desire. On this the individual will eat as much as it can—thus learning pro tanto to be able to eat more, and to want more under ordinary circumstances—and will also breed rapidly up to the balance of the abundance. This is the work of the agencies incorporate in the organism, and will bring desire level with power again. Famine, on the other hand, puts desire ahead of power, and the incorporate agencies must either bring power up by resource and invention, or must pull desire back by eating less, both as individuals, and as the race, that is to say, by breeding less freely; for breeding is an assimilation of outside matter so closely akin to feeding, that it is only the feeding of the race, as against that of the individual.
I do not think the reader will find any clearer manner of picturing to himself the development of organism than by keeping the normal growth of wealth continually in his mind. He will find few of the phenomena of organic development which have not their counterpart in the acquisition of wealth. Thus a too sudden acquisition, owing to accidental and external circumstances and due to no internal source of energy, will be commonly lost in the next few generations. So a sudden sport due to a lucky accident of soil will not generally be perpetuated if the offspring plant be restored to its normal soil. Again, if the advance in power carry power suddenly far beyond any past desire, or be far greater than any past-remembered advance of power beyond desire—then desire will not come up level easily, but only with difficulty and all manner of extravagance, such as is likely to destroy the power itself. Demand and Supply are also good illustrations.