24. Between the vegetable and animal kingdom there are no distinctly traced boundaries. Upon the borders of the two are the zoophytes, or animal plants, of which the name indicates that they belong to both: they are the hyphen between the two.
Like animal, plants are born, live, grow, are nourished, breathe, reproduce their kind, and die. Like them they have need of light, heat, and water; if they are deprived of them, they wither and die. The absorption of vitiated air and deleterious substances poisons them. Their distinctive trait of character, the most defined, is of being attached to the soil, and, without leaving their place, drawing their nourishment from it.
The zoophyte has the exterior appearance of a plant. Like the plant it belongs to the soil, but seems to partake more of the nature of an animal. It draws its nourishment from the ambient midst.
An animal, being one degree above a zoophyte, is free to go and seek its food. Firstly, there are innumerable varieties of polyps with gelatinous bodies, without very distinct organs, and which differ from plants only by locomotion. Then come in the order of development those with organs of vital activity and instinct, — intestinal worms, mollusks, fleshly animals without bones, of which some are entirely destitute, as slugs or cuttle-fish; others are provided with shells, as snails and oysters; then shell-fish, of which the skin is invested with a hard shell, like crabs and lobsters; insects, who lead a very active life, and manifest an industrious instinct, like the ant, the bee, and the spider; a few submit themselves to a metamorphosis, as the caterpillar, which is transformed into an elegant butterfly. Then comes the order of vertebrates, — animals with a bony framework, — which comprises fish, reptiles, birds, and mammals, of which the organization is more complete.
25. If we consider only the two opposite ends of the chain, there is no apparent analogy in these beings. However, if we go from one ring to the other, without solution of continuity, we arrive, without any sudden transition, from plants to vertebral animals. One can then understand the possibility that animals of complex organization may be no more than a transformation, or if we prefer, a gradual development, unnoticeable at first, of the immediately inferior specie and thus successively down to the most elementary primitive being. Between the acorn and the oak tree, there is a great difference; nevertheless, if we follow step by step the development of the acorn, we will arrive at the oak tree, and then we will not be surprised to see that it originated from such a small seed. If the acorn encompasses, in latency, the elements appropriate to the formation of a gigantic tree, why then won’t the same happen from the insect to the elephant? (n° 23)
From the above we conclude that there is no spontaneous generation, except for elementary organic beings; the superior species would be a product of successive transformations of these same beings, achieved as soon as atmospheric conditions were propitious for it. When each species acquired the ability to reproduce, their crossbreeding brought about innumerable varieties. Then, once the species were set in conditions of lasting vitality, who could say that the primitive germs from which they emerged did not disappear, for lack of usefulness? Who could say that our current day insect is not the same that, from transformation to transformation, produced the elephant? This would explain why there is no spontaneous generation among animals of complex organization.
Although not yet admitted as final, this theory tends evidently to prevail in Science today. It is the theory accepted by most serious observers, for being the most rational.