barnacles and plants, Darwin struggled with defining species and varieties under this
system. Overall he believed that genealogy should be the basis of a taxonomic
system (Padian 1999). He frequently expressed frustration when dealing with taxonomy. He found that the use of morphological characteristics to define species was
necessary and recognized that such factors could help reveal genealogy. He wrote to
his friend, zoologist Thomas Huxley, on September 26, 1875:
In regard to Classification, & all the endless disputes about the ‘Natural System’ which no
two authors define in same way, I believe it ought, in accordance to my heteredox notions, to
be simply genealogical.—But as we have no written pedigrees, you will, perhaps, say this
will not help much; but I think it ultimately will, whenever heteredoxy becomes orthodoxy,
for it will clear away an immense amount of rubbish about the value of characters &—will
make the difference between analogy & homology, clear.—The time will come I believe,
though I shall not live to see it, when we shall have very fairly true genealogical trees of each
great kingdom of nature. (Darwin Correspondence Project Letter no. 2143 2020b)
The use of characters to demark taxonomic differences among microorganisms was
hindered by the limited number and often indistinct nature of their characters. Early
attempts to describe the characters of microbes were thwarted by defects in the
optical properties of early microscopes. Ehrenberg, for example, stated, “The infusorian has the same sum of organization-systems as a man.” This reflected his dislike
of systems that place organisms in hierarchies of apparently “simple” organisms at
the bottom and “higher” ones at the top (Churchill 1989). He claimed that protozoa
(“Polygastrica”) had circulatory, digestive, excretory, motor, sexual, and nervous
systems. For example, he believed that the nucleus was their stomach. Contemporaries of his, however, recognized the nucleus as not an organ, but a structure
somehow involved in cell reproduction. A homology between the nuclei of singlecelled organisms and those of animal cells was increasingly recognized. This
culminated, by the 1870s, to the acceptance of single-celled protozoans as a taxonomic group of organisms separate from multicellular organisms.
The place of bacteria in this taxonomy was, however, still problematic. In 1838,
Ehrenberg placed them in with the protozoans as the groups Monadina and Vibrionia,
in line with a 1786 classification by Otto Friedrich Müller (Drews 2000). Theodor
von Siebold moved them to the plants in 1845 on the grounds that their movements
were involuntary (Churchill 1989). There they stayed until Ernst Haeckel in 1866
divided the animal kingdom into the Protozoa and Metazoa and included the Monera
as one of eight divisions of the Protozoa. Monera were described as the “most simple
organisms, without structure, homogeneous pieces of Plasma,” and bacteria known as
Vibrio were given as an example (Kutschera 2016).
The introduction of solid culture media in the 1870s and especially the development of the method of streaking inocula to get single colonies by Robert Koch in
1877 lead to the concept of stable phenotypes in bacteria. At that time several
investigators held to the idea that bacteria developed different shapes depending
upon the conditions of their growth. Consequently, some taxonomies lumped all
bacteria into a single genus or even species (Drews 2000). Ferdinand Cohn, a student
of Ehrenberg, at that time began to characterize “form-genera” and “form-species”
based on the stable morphologies provided by pure cultures. As mentioned previously, Cohn was not only aware of Darwin’s work, but was a correspondent of his
26
K. M. Noll
system. Overall he believed that genealogy should be the basis of a taxonomic
system (Padian 1999). He frequently expressed frustration when dealing with taxonomy. He found that the use of morphological characteristics to define species was
necessary and recognized that such factors could help reveal genealogy. He wrote to
his friend, zoologist Thomas Huxley, on September 26, 1875:
In regard to Classification, & all the endless disputes about the ‘Natural System’ which no
two authors define in same way, I believe it ought, in accordance to my heteredox notions, to
be simply genealogical.—But as we have no written pedigrees, you will, perhaps, say this
will not help much; but I think it ultimately will, whenever heteredoxy becomes orthodoxy,
for it will clear away an immense amount of rubbish about the value of characters &—will
make the difference between analogy & homology, clear.—The time will come I believe,
though I shall not live to see it, when we shall have very fairly true genealogical trees of each
great kingdom of nature. (Darwin Correspondence Project Letter no. 2143 2020b)
The use of characters to demark taxonomic differences among microorganisms was
hindered by the limited number and often indistinct nature of their characters. Early
attempts to describe the characters of microbes were thwarted by defects in the
optical properties of early microscopes. Ehrenberg, for example, stated, “The infusorian has the same sum of organization-systems as a man.” This reflected his dislike
of systems that place organisms in hierarchies of apparently “simple” organisms at
the bottom and “higher” ones at the top (Churchill 1989). He claimed that protozoa
(“Polygastrica”) had circulatory, digestive, excretory, motor, sexual, and nervous
systems. For example, he believed that the nucleus was their stomach. Contemporaries of his, however, recognized the nucleus as not an organ, but a structure
somehow involved in cell reproduction. A homology between the nuclei of singlecelled organisms and those of animal cells was increasingly recognized. This
culminated, by the 1870s, to the acceptance of single-celled protozoans as a taxonomic group of organisms separate from multicellular organisms.
The place of bacteria in this taxonomy was, however, still problematic. In 1838,
Ehrenberg placed them in with the protozoans as the groups Monadina and Vibrionia,
in line with a 1786 classification by Otto Friedrich Müller (Drews 2000). Theodor
von Siebold moved them to the plants in 1845 on the grounds that their movements
were involuntary (Churchill 1989). There they stayed until Ernst Haeckel in 1866
divided the animal kingdom into the Protozoa and Metazoa and included the Monera
as one of eight divisions of the Protozoa. Monera were described as the “most simple
organisms, without structure, homogeneous pieces of Plasma,” and bacteria known as
Vibrio were given as an example (Kutschera 2016).
The introduction of solid culture media in the 1870s and especially the development of the method of streaking inocula to get single colonies by Robert Koch in
1877 lead to the concept of stable phenotypes in bacteria. At that time several
investigators held to the idea that bacteria developed different shapes depending
upon the conditions of their growth. Consequently, some taxonomies lumped all
bacteria into a single genus or even species (Drews 2000). Ferdinand Cohn, a student
of Ehrenberg, at that time began to characterize “form-genera” and “form-species”
based on the stable morphologies provided by pure cultures. As mentioned previously, Cohn was not only aware of Darwin’s work, but was a correspondent of his
26
K. M. Noll