Flora and Fauna within Living Animals (1853)
Completed in 1850 and published three years later, "A Flora and Fauna within Living Animals" (1) was the most significant of Joseph Leidy's early publications in parasitology. Joseph Henry, then Secretary of the Smithsonian Institution, commented that: "The whole forming the most remarkable paper on physiology by one of our countryman." Henry B. Ward, a prominent American parasitologist of the early 20th Century, considered the book to be "epoch making" and commented that Leidy's anatomical descriptions and illustrations were "unequaled in the literature of the time."
Three species entophyta and entozoa found in the gut of a millipede. The figure on the left is the fungus Enterobrysus attenuatus. Seven individuals of the nematode Ascaris infecta attached to the gut wall of the host are shown in the center. The figure on the right show the front end of another nematode, Thelastomum appendiculatum. These three figures are details from two lithographs presented in "A Flora and Fauna within Living Animals". Click on the figures to view the full lithographs or view Plate IV and Plate VII
This 67 page publication presents the results of an intensive study Leidy of the entofauna and entoflora (2) in the gut of a species of millipede and a beetle. He demonstrated that these two invertebrate species housed a characteristic fauna and flora. He described 8 species of entophytes (fungi and bacteria) and 12 species of entozoa (mostly nematodes). To varying degrees, he also worked out their developmental stages and and their associations with particular organs of the host.
In some respects, the introduction is the most remarkable part of this monograph. It introduces the science of parasitology (or the study of endosymbionts) to American readers, discusses the evolution of life, and puts forward a scathing rebuttal to the idea of spontaneous generation.
Leidy's introduction to parasitology includes a list of general principles. He states that all animals naturally harbor a variety of parasites, but most of them are harmless (3). He notes that most parasites are found within the host's gut and that intracellular parasites are rare (4). He also notes that aquatic animals tend to have more parasites because water is a better medium for transmission than air, and that terrestrial animals are more likely to be infested with ectoparasites (parasites on the skin).
Leidy's discussion on the evolution of life predates Darwin's On the Origin of Species by nearly a decade. In it he grapples with the puzzle of extinction and the "indispensable conditions of action" that give rise to new species. Leidy also envisions an evolutionary progression "from the oscillating particles of organic matter to a Bacterium; from this to a Vibrio; then to a Monad, and so gradually up to the the highest orders of life." Although it wasn't explicitly atheistic, his account of the evolution of life did not invoke creation by a divine being. This led some influential people to oppose Leidy's appointment as the Professor of Anatomy at the Medical School of the University of Pennsylvania.
One of Leidy's most emphatic passages in "A Flora and Fauna within Living Animals" is his rebuttal of idea of Spontaneous Generation. This idea —supported by some of the leading scientists of his day— proposed that given the right conditions, microbes and a variety of minute animals were literally created (or appeared out of nothing). Leidy noted that "The entozoa have always constituted the strongest support for the doctrine of equivocal or spontaneous generation" because of the complexity of their life cycles. In other words, entozoa seem to appear spontaneously because: "their various stages of existence are passed under totally different circumstances; sometimes within one organ and then another of the same animal; sometimes in several animals; and, at other times, even quite external to and independent of the animals they infest."
But Leidy challenged the proponents of spontaneous generation to provide one single direct observation to substantiate the phenomenon. His observations of the entozoa of the millipede and beetle hosts revealed the presence of reproductive organs, eggs and different stages of development. Although the complete life histories of many parasites continued to elude him and other researchers, Leidy concluded that:
"The very great majority of modern observations indicate that entozoa and entophyte are produced from germs derived from parents and have a cyclical development."
Leidy wasn't the only scientist trying to discredit the idea of Spontaneous Generation, but he was major voice in the United States. The most famous and important critic was Louis Pasteur, who developed the germ theory of disease.
Two lithographic plates from "Flora and Fauna within Living Animals" (1853) are presented here:
Websites:
- AccessExcellence's
web page on Louis Pasteur:
www.accessexcellence.org/AB/BC/Louis_Pasteur.html - AccessExcellence's web page on Spontaneous Generation:
www.accessexcellence.org/RC/AB/BC/Spontaneous_Generation.html
Notes:
- A
Flora and Fauna within Living Animals." Smithsonian Contributions to Knowledge.
Vol. 5. 1853.
[go back] - Entoflora
(=inside plant) are plants, fungi and bacteria that live inside the body of an
animal. (Fungi and bacteria were once considered to be types of plants.) Entozoa
are animals and protozoa that live inside of animals.
[go back] -
Scientists during Leidy's
time used the term parasite in reference to any animal living on or within another
animal (the host). The modern usage is applied only to those animals that feed
on their host. Organisms that live within an animal, but don't harm it are now
often called endosymbionts.
Even if they are actually feeding off the host, some parasites are not very harmful. This apparently results from the disadvantage of harming your host. If the host dies, you loose your host. The trick is to keep your host living long enough for you to reproduce and spread your offspring.
On the other hand, some parasites are very harmful. Malaria, sleeping sickness, river blindness, and Chaga's disease are examples of particularly harmful parasites. Significantly, these and many other harmful parasitic diseases are transmitted by biting insects such as mosquitos and black flies.
[go back] - Intracellular parasites reside within the cells of its host. Leidy and his
European peers rarely found intracellular parasites because they lacked
the microscopic techniques to reveal them. One consequence of this technical shortcoming
is that and others missed the most serious human parasite of all, malaria. Staining
techniques for the malarial parasite (Plasmodium) weren't developed until
1890. The phase contrast microscope, another vital tool for viewing intracellular
parasites wouldn't be available until much later.
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