Theory of Panspermia: An Idea that Will Not Die

Ancient Greek philosopher Aristotle (384-322 B.C.E.) decreed that all creatures that do not move come into being “by the spontaneous action of Nature, arising either from decomposition of the earth or of some parts in other plants.” (1) His predecessor Anaxagoras (500-428 B.C.E.) disagreed, declaring that seeds of plant and animal life are inherent in the cosmos and take root whenever conditions become favorable. (2) Anaxagoras called his belief panspermia, from the Greek panspermi, a mixture of all seeds: pan-, pan- + sperma, seed. (3)

Aristotle’s influence on Western thought for the next two millennia was pervasive. Anaxagoras’ theory of panspermia waited an astonishing 2,150 years following Aristotle’s death to resurface in the mid-1800s of the current era when it still faced opposition from modern-day Aristotelians, also known as Darwinians (more below). (4)

Nineteenth Century: A Hospitable Time for Panspermia

Three new theories emerged in the eighteenth and nineteenth centuries, which yielded fertile soil for reemergence of the theory of panspermia. First, the German philosopher Immanuel Kant (1724-1804) and later the French mathematician Pierre-Simon LaPlace (1749-1827), using more powerful telescopes and spectrometers, published their theories on the origin of the solar system. They proposed that the sun and planets formed from a slowly rotating gas and dust dark nebula, which gradually collapsed and flattened due to gravity. (6) Their theories implied that in the early stages of geological history the Earth’s surface was far too hot to sustain living organisms. The origin of terrestrial life was not contemporaneous with the origin of the Earth. Life, they said, “must have made its first appearance in the absence of pre-existing organisms.” (5)

Dark nebula in red square against starry sky. Source: http://www.astro.caltech.edu/palomar/images/b92square.jpg; accessed August 14, 2007.	Dark nebula without red square. Source: http://www.astro.caltech.edu/palomar/images/b92.jpg; accessed August 14, 2007.

Second, the English naturalist Charles Robert Darwin (1809-1882) espoused the theory of evolution first published in Origin of Species in 1859. He contended that all species of life have evolved over time from one or a few common ancestors through the process of natural selection. He invoked no divine intervention, vital forces or teleological concepts. “There was an omission in the theory, however, in so far as it demanded some starting point, some type of primordial cell from which all present forms of life had ultimately descended.” (2) Darwin remained aloof from this issue, except once when he briefly broke his silence in a famous letter to his friend Joseph Dalton Hooker (1817-1911) in 1871:

“It is often said that all the conditions for the first production of a living organism are present, which could ever have been present. But if (and Oh! what a big if!) we could conceive in some warm little pond [italics by author], with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.” (7)

Darwin’s “warm little pond” idea would reverberate for the next century and a half. It set the stage for the theory of chemical evolution as the origin of life, which still is the most believed theory by most Western scientists.

Louis Pasteur. Source:  http://www.pasteur.fr/pasteur/musees/museesUS/im/pasteur.jpg; accessed August 14, 2007.	Charles Darwin. Source:  http://upload.wikimedia.org/wikipedia/en/3/3e/Charles_Darwin_1854.jpg; accessed August 14, 2007.

The illustrious French chemist and microbiologist Louis Pasteur (1822-1895) proposed the third theory. He took aim at the theory of spontaneous generation, which proposed that decaying organic substances beget living organisms, e.g. mice and maggots spontaneously appear in stored grain and meat, respectively. Pasteur, in a set of famous experiments, proved that all life comes from previous life and never from non-life. He filled glass flasks containing sterile culture medium (broth) with air from the Alps and from the streets of Paris. (8) Flasks opened in the Paris streets grew abundant organisms while flasks opened in the high mountains remained sterile with rare exception. Pasteur summarized his findings in the essay titled “Sur les corpuscles organiés qui existent dans l'atmosphère. Examen de la doctrine des générations spontanées” (“On the organized bodies which exist in the air. Examination of the doctrine of spontaneous generation.”) (9)

In response to proponents of spontaneous generation, Pasteur told his students during a lecture at the Sorbonne in 1864:

“Gentlemen, I could point to that liquid (in the flask of sterile culture medium on the table before him) and say to you, I have taken my drop of water from the immensity of creation, and I have taken it full of the elements appropriated to the development of inferior beings. And I wait, I watch, I question it, begging it to recommence for me the beautiful spectacle of the first creation. But it is dumb, dumb since these experiments were begun several years ago; it is dumb because I have kept it from the only thing man cannot produce, from the germs which float in the air, from Life, for Life is a germ and a germ is Life. Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment.

“No, there is now no circumstance known in which it can be affirmed that microscopic beings came into the world without germs, without parents similar to themselves. Those who affirm it have been duped by illusions, by ill-conducted experiments, spoilt by errors that they either did not perceive or did not know how to avoid.” (10)

Darwinism’s Clash with Pasteurism

Darwin implied in his letter to Hooker that life could have come from non-life (spontaneous generation) in the warm little pond. Pasteur, by contrast, demonstrated scientifically that life is necessarily antecedent to life, and never arises via spontaneous generation. How could the two views co-exist? Panspermia provided a solution to the dilemma. In the context of panspermia, life appeared on Earth from the universe at large when the conditions on Earth were ripe. The most important question for panspermists became, what were the mechanisms by which life reached Earth from other planets or celestial bodies? (11)

Physicists and Astronomers Champion Panspermia

French astronomer and panspermist Camille Flammarion (1842-1925) deeply influenced German physician Hermann E. Richter who reviewed advances in a wide range of scientific areas and of explaining their relevance to medicine in 1864. (12,13) Flammarion believed in the plurality of inhabited worlds, as described in his works La Pluralité des Mondes Habitées (The Plurality of Habitable Worlds) (1862) and Les Mondes Imaginaires et Les Mondes Réels (Real and Imaginary Worlds) (1864).

The former book was “a very successful study of the habitability of the planets of the solar system, going through 36 editions by 1892, and translated into at least six and possibly as many as fifteen languages. The latter “book was a thorough history and critical survey of explorations in extraterrestrial life both in philosophy and in the cosmic voyage genre.  In its pages Flammarion admits the inspiration of Kepler, Godwin, Cyrano de Bergerac and Jonathan Swift in literary speculations on the habitability of planets, and even stars.” (14)

Richter wrote an important review article on Darwinism in 1865 titled “Zur Darin’schen Lehre” in the medical yearbook titled Schmidt’s Jahrbucher der in- und auslandischen gesammten Medicin (Volume 126, pp. 243-249). Richter praised Darwin’s “notion of the transformation of species by natural selection” but criticized Darwin’s theory of evolution for failing “to provide an explanation of the origin of the primordial cell type from which subsequent life forms had descended.” (12) “Richter could not envisage how living organisms could have arisen from purposeless inorganic matter [in Darwin’s warm little pond] without the intervention of some teleological principle beyond natural law.” (12)

To address this problem, Richter famously wrote: “We therefore also regard the existence of organic life in the universe as eternal; it has always exited and has propagated itself in uninterrupted succession, and indeed in organized form – not as a mysterious Urschleim, but in the form of living organisms, as cells or individuals composed of cells. ‘Omne vivum ab aeternitate e cellula!’” (15) He proposed that among particles moving about in space, like meteors, are the germs of microscopic organisms (he called them cosmozoa) capable of establishing life on the earth. The first organisms on Earth came from space. “If only one such organism fell on a planet with conditions suitable for life, it could become the starting-point for the entire process of evolution,” he said. (15)

Richter’s theory solved many problems about the origin of life on Earth. His theory made the theory of spontaneous generation irrelevant and accommodated Darwin’s theory of evolution without having to explain the origin of primordial cells from chemicals or inorganic matter on a lifeless Earth. Furthermore, his theory did not require supernatural intervention and, “by declaring all organisms to have descended always from other living beings, retained the autonomy of life.” (15)

Irish-Scotsman physicist Sir William Thompson (First Baron Kelvin, Lord Kelvin) (1824-1907) developed a related theory about panspermia, which he shared in a speech before theForty-First Meeting of the British Association for the Advancement of Science held at Edinburgh in August 1871, excerpted as follows:

“How, then, did life originate on the Earth? Tracing the physical history of the Earth backwards, on strict dynamical principles, we are brought to a red-hot melted globe on which no life could exist [Recall that this was the theory advanced by Kant and LaPlace, noted above]. Hence when the Earth was first fit for life, there was no living thing on it. There were rocks solid and disintegrated, water, air all round, warmed and illuminated by a brilliant Sun, ready to become a garden. Did grass and trees and flowers spring into existence, in all the fullness of ripe beauty, by a fiat of Creative Power? or did vegetation, growing up from seed sown, spread and multiply over the whole Earth? Science is bound, by the everlasting vow of honour, to face fearlessly every problem which can be fairly presented to it. If a probable solution, consistent with the ordinary course of nature, can be found, we must not invoke an abnormal act of Creative Power. [Italics by author.] When a lava stream flows down the sides of Vesuvius or Etna it quickly cools and becomes solid; and after a few weeks or years it teems with vegetable and animal life, which for it originated by the transport of seed and ova and by the migration of individual living creatures. When a volcanic island springs up from the sea, and after a few years is found clothed with vegetation, we do not hesitate to assume that seed has been wafted to it through the air, or floated to it on rafts. Is it not possible, and if possible, is it not probable, that the beginning of vegetable life on the Earth is to be similarly explained? Every year thousands, probably millions, of fragments of solid matter fall upon the Earth—whence came these fragments? What is the previous history of any one of them? Was it created in the beginning of time an amorphous mass? This idea is so unacceptable that, tacitly or explicitly, all men reject it. It is often assumed that all, and it is certain that some, meteoric stones are fragments which had been broken off from greater masses and launched free into space. It is as sure that collisions must occur between great masses moving through space as it is that ships, steered without intelligence directed to prevent collision, could not cross and recross the Atlantic for thousands of years with immunity from collisions. When two great masses come into collision in space it is certain that a large part of each is melted; but it seems also quite certain that in many cases a large quantity of debris must be shot forth in all directions, much of which may have experienced no greater violence than individual pieces of rock experience in a land-slip or in blasting by gunpowder. Should the time when this earth [sic] comes into collision with another body, comparable in dimensions to itself, be when it is still clothed as at present with vegetation, many great and small fragments carrying seed and living plants and animals would undoubtedly be scattered through space. Hence and because we all confidently believe that there are at present, and have been from time immemorial, many worlds of life besides our own, we must regard it as probable in the highest degree that there are countless seed-bearing meteoric stones moving about through space. If at the present instant no life existed upon this earth, one such stone falling upon it might, by what we blindly call natural causes, lead to its becoming covered with vegetation. I am fully conscious of the many scientific objections which may be urged against this hypothesis; but I believe them to be all answerable. I have already taxed your patience too severely to allow me to think of discussing any of them on the present occasion. The hypothesis that life originated on this earth through moss-grown fragments from the ruins of another world may seem wild and visionary; all I maintain is that it is not unscientific.” (16)

In this excerpt, Kelvin cites great and small “fragments” (i.e., panspermia) falling to Earth as a natural solution to the question of the first appearance of life on Earth, without invoking a supernatural intervention. Kelvin also advanced a mechanism for the distribution in all directions of the individual pieces of rock bearing the beginnings of life—collisions between celestial bodies that resulted in meteors dropping to Earth as life-bearing meteorites. (17) Kelvin disputed Darwin’s theory of evolution, “if evolution there has been”, he noted, based on the concept of natural selection. To Kelvin, “this concept took into acount insufficiently the argument of design. Kelvin ended his address by invoking a Creator, as follows:

“But overpoweringly strong proofs of intelligent and benevolent design lie all around us, and if ever perpelxities, whether metaphysical or scientific, turn us away from them for a time, they come back upon us with irresistible force, showing us through nature the influence of a free will, and teaching us that all living beings dependent upon one everacting Creator and Ruler.” (16)

Thus, Kelvin espoused a theory of panspermia dependent on the mystery and miracle of an everacting Creator and Ruler, a unique blend of science and transcendentalism.

The German biologist Ferdinand Cohn (1828-1898) of Breslau first determined from careful observation of bacteria under his microscope that a morphological classification of them was possible. In his 1881 book titled Bacteria, the Smallest Living Things, he wrote the following lines:

“[T]he smallest, and at the same time the simplest and lowest of allliving forms, we call Bacteria.* Bacteria form the boundary line of life; beyond them life does not exist, so far as we can tell  with our most powerful microscopes…If we could view a person under a microscope suitable for examining bacteria, the person would appear as large as Mont Blanc. But even under this extreme magifncaiton, the smallest bacteria do not appear larger thatn the periods and commas on a printed page. These smallest bacteria may be compared with a human as a grain of sand is to Mont Blanc…We are able to discern nothing about the internal structures of bacteria, and even their existence would be for the most part be hidden, if they did not live in such large masses. ..Bacteria do not generate the material that forms their bodies de novo, but take it from the environment as food. Therefore, no more bacteria can be formed than the food available…Bacteria are among the most widespread of organisms. They are are omnipresent, in air or water, attached to surfaces, but they only develop into masses when decomposition, corruption, fermentation, or putrefaction can take place…In recent times, our knowledge of the effects which bacteria can have over the life and death of humans has been revealed…All epidemics, chlolera, pestilence, typhus, diphtheria, variola, scarlet fever, hospital gangrene, epizootic, and the like, have certain features in common. These diseases do not arise de novo, but are introduced from another place where they have been prevalent, by means of a diseased person or through material which has been in contact with such: they spred only through contagion.” (18,19)

*”From the Greek word for a little staff or rod.”

Cohn, like Pasteur, vigorously opposed the concept of spontaneous generation, which his own discovery of heat-resistant bacterial spores had helped to refute, and supported Lord Kelvin’s idea that life came from another inhabited planet. (20) Cohn wrote in 1881,

“[I]n the development of bacteria the key will be found to the origin of life in the world in general. [Italics by author]…Now it is very certain that  life had a beginning on the earth, but how did the first living beings originate? For this all analogy is wanting…According to our present knowledge life may be compared to the holy fire of Vesta, which is eternally maintained, through the kindling of the new brand from the old…[L]ife on this earth, not having originated of itself, must therefore have been conveyed to our world from another. We know that numberless meteoric stones which have fallen to the earth, were once independent bodies or parts of such. In certain meteorites carbon, and certain combinations containing carbon, have been found, which points to organic formation. It is possible to think, that at somet time a germ, with life and capacity for development, could have survived the glowing heat which generally accompanies the entrance of a new comer from space into our atmosphere, and that from such a germ all living beings might have descended. Thus, some time the commencement of life may have descended from Heaven upon this lifeless earth; as according to the myth, the living spark was brought down by Prometheus form Olympia.” (20)

German physicist Hermann von Helmholtz (1821-1894) championed the Kant-LaPlace nebular hypothesis about which he lectured in Europe in 1871. Von Helmholtz noted that meteorites sometimes contained hydrocarbons and that the light of the heads of comets exhibited spectra resembling those of gases containing hydrogen and carbon. Thus, he concluded,

“But carbon is the element, which is characteristic of organic compounds, from which living bodies are built up. Who knows whether these bodies, which everywhere swarm through space, do not scatter germs of life wherever there is a new world, which has become capable of giving a dwelling place to organic bodies? And this life we might perhaps consider as allied to ours in its primitive germ, however different might be the form which  it would assume in adapting itself to its new dwelling place.” (20)

In 1874, Helmholtz wrote:

“It appears to me to be a fully correct scientific procedure, if all our attempts fail to cause the production of organisms from non-living matter, to raise the question whether life has ever arisen, whether it is not just as old as matter itself, and whether seeds have not been carried form one planet to another and have developed everywhere where they have fallen on fertile soil.” (21)

Earth: a Closed or an Open System? Darwinians and Panspermists Disagree

Nineteenth century biologists in general and with few exceptions (e.g., Ferdinand Cohn) insisted that life emerged in the closed (Aristotelian) system that is the planet Earth. (22) Darwinians were particularly attached to this worldview. Meanwhile, nineteenth century (non-biologist) physical scientists, i.e., physicists and astronomers, insisted that life is fundamental to the universe and arrived to Earth in an open system that includes all of the universe. The clash between the theories was intense. Irish physicist John Tyndall (1820-1893) on January 21, 1870, presented a lecture at the Royal Institution, London, in which he reported the presence everywhere in the atmosphere of small organic particles. An anonymous writer in the newly founded British weekly science magazine Nature (first published on November 4, 1869, still very much around today) denounced his ideas in an article titled “The atmospheric-germ theory”. (22)

“The anonymous writer was evidently a strong supporter of Darwin, propaganda for whom seems to have been the main objective of the magazine, at any rate in its early days,” remarked two twentieth century panspermists, Fred Hoyle and Chandra Wickramasinghe. (22) The article has many catastrophic mistakes, they said, including disparaging the validity of the germ theory of disease and Pasteur’s work on fermentation and on silkworm disease. (23) They continue:

“What Darwinians could see, even as early as 1870, was that if microscopic units of life existed outside ordinary plants and animals, there was the possiblity that changes had been induced in plants and animals by invasion from without. Evolution might then become more a question of invasion than of mutation occuring internally, in which case Darwin’s cherished precept, natura nonfacit saltum (nature does not go in jumps) would be in danger. In the interest of preserving their dogmas, the early Darwinians were evidently prepared to deny bacteriology.” (22)

The anonymous writer in Nature castigated the “insane individuals who thought that units of life might exist, not just terrestrially, but cosmically.” Among this group of “insane individuals” were Cohn, Helmholtz, and Kelvin (see above)!

Svante Arrhenius Takes Panspermia into the Twentieth Century

Swedish physical chemist Svante Arrhenius (1859-1927) straddled two centuries. Did he espouse the closed (Darwinian) or open system theory of the origin of life? His credential as a physical chemist indicates the latter. In his famous book titled Worlds in the Making, first published in English in 1908, he presented a new version of Richter’s theory of cosmozoa or, as he termed it, panspermia. (25)

Arrhenius summarized the previous century’s work for twentieth century denizens. He wrote:

“The researches of Pasteur, in particular, and the methods of sterilization which are based upon them and which are applied every day in bacteriological laboratories, have more and more forced the conviction upon us that a germ is indispensable for the origination of life. And yet eminent scientists take up the pen again and again in order to demonstrate the possiblity of the “Generatio spontanea.” In this they do not rely on the safe methods of natural science, but they proceed on philosophical lines of argument. Life, they suggest must once have had a beginning, and we are hence forced to believe that spontaneous generation, even if not realizeable under abiotic conditions, must have once occurred.” (26)

Arrhenius continued,

“Almost every year the statement is repeated in biological literature that we have at last succeeded in producing life from dead matter…We fully share the opinion which the great natural philosopher Lord Kelvin has expressed in the following words: ‘A very ancient speculation, still clung to by many naturalists (so much so that I have a choice of modern terms to quote in expressing it), supposes that, under meteorological conditions very different from the present, dead matter may have run together or crystallized or fermented into ‘germs of life,’ or ‘organic cells,’ or ‘protoplasm’ [i.e., in Darwin’s warm little pond]. But science brings a vast mass of inductive evidence against this hypothesis of spontaneous generation. Dead matter cannot become living without coming under the influence of matter previously alive. This seems to me as sure a teaching of science as the law of gravitation.” (25)

Arrhenius supported the theory of panspermia. “The so-called theory of panspermia really shows a way,” he said. “According to this theory life-giving seeds are drifting about in space. They encounter the planets, and fill their surfaces with life as soon as the necessary conditions for the existence of organic beings are established…” (25)

Arrhenius acknowledged H.E. Richter as the founder of the modern theory of panspermia, who “attempted to supplement the doctrine of Darwin by combining the conception of panspermia with it. Flummarian’s book on the plurality of inhabited worlds suggested to Richter the idea that seeds had come from some other inhabited world to our earth. He emphasized the fact that carbon has been found in meteorites which move in orbits similar to those of the comets which wander about in space; and in this carbon he sees the rests of organic life.”

Arrhenius lauded Lord Kelvin’s work and discoursed on the mechanisms of interplanetary transfer of spores, but ends his Worlds in the Making on a cautious note.  (26) “There is little probability, though, of our even being able to demonstrate the correctness of this view by an examination of seed falling down upon our earth.” (27)

Death of Panspermia Again?

After Arrhenius, the idea of panspermia lost momentum, according to historian Kamminga. (26) Its problem was twofold. First, physical scientists despaired of ever having the tools to establish its validity. In 1897, for example, astronomer Simon Newcomb (1835-1909) wrote:

“Slow indeed is progress in the solution of the greatest problems when measured by what we want to know. Some questions may require centuries, others thousands of years for their answer. And yet never was progress more rapid than during our time. In some directions our astronomers of today are out of sight of those of fifty years ago; we are even gaining heights which, twenty years ago, looked hopeless. Never before had the astronomer so much work, good, hard, yet hopeful work before him as today.” (28)

Second, Darwinian Marxist scientists, such as A. I. Oparin and J.B.S. Haldane, and American scientists Stanley L. Miller and Harold C. Urey at the University of Chicago, believed life emerged spontaneously via chemical evolution in a warm little pond on Earth. The latter two conducted an experiment in support of their hypothesis. As a result, for the first half of the twentieth century the Darwinian idea of life emerging spontaneously on Earth from an input of energy into a warm little pond filled with inorganic molecules trumped the idea that life is fundamental to the universe, and seeded Earth as conditions became ripe.

Notes:

1. Aristotle did acknowledge, “Now some animals come into being from the union of male and female, i.e. all those kinds of animal which possess the two sexes.” Source: Aristotle: On the Generation of Animals (Arthur Platt, translator). Book 1, p. 1. eBooks@Adelaide, 2007. Available at http://etext.library.adelaide.edu.au/a/aristotle/generation/book1.html; accessed August 6, 2007.
2. Harmke Kamminga: “Historical perspective: The problem of the origin of life in the context of developments in biology.” Origins of Life and Evolution of the Biosphere, 1988, Volume 18, pp. 1-11. Abstract available at http://www.springerlink.com/content/t5845j3u237h2700/; accessed August 7, 2007.
3. Etymology of panspermia is from Bartlesby.com at http://www.bartleby.com/61/62/P0046250.html; accessed August 1, 2007.
4. Fred Hoyle and Nalin Chandra Wickramasinghe: The Theory of Cosmic Grains. Kluwer Academic Publishers, 1991, pp. 169.
5. Harmke Kamminga: “Life from space: A history of panspermia.” Vistas in Astronomy, 1982, Volume 26, p. 67.
6. USGS CMG InfoBank: “Earth Condensed From a Nebula”. Available at http://walrus.wr.usgs.gov/infobank/programs/html/school/moviepage/02.01.08.html; accessed August 7, 2007.
7. David Darling: “Charles Robert Darwin”. Available at http://www.daviddarling.info/encyclopedia/D/DarwinC.html; accessed August 7, 2007.
8. John L. Wilson: Stanford University School of Medicine and the Predecessor Schools: An Historical Perspective. Chapter 5: “Louis Pasteur (1822-1895)”. Available online at http://elane.stanford.edu/wilson/Text/5f.html#Ref139; accessed August 6, 2007.
9. Fielding H. Garrison: History of Medicine, W.B. Saunders, 1913, pp. 518-520.
10. John H. Mann: Louis Pasteur: Founder of Bacteriology. Charles Scribner’s Sons, 1964, p. 60.
11. Florence Raulin-Cerceau, Marie-Christine Maurel, Jean Schneider: “From panspermia to bioastronomy, the evolution of the hypothesis of universal life.” Origins of Life and Evolution of the Biosphere, 1998. Volume 28, p. 598.Abstract available at http://www.springerlink.com/content/m1t14rtr7372tp22/; accessed August 7, 2007.
12. Harmke Kamminga: “Life from space: A history of panspermia.” Vistas in Astronomy, 1982, Volume 26, p. 68.
13. Steven J. Dick: The Biological Universe: The Twentieth Century Extraterrestrial Life Debate and the Limits of Science, 1999. Cambridge University Press, p. 326.
14. “French Tales of Infinity”. European Edition Astrobiology Magazine. Available at http://www.astrobio.net/amee/spring_2007/retrospections.htm; accessed August 7, 2007.
15. Harmke Kamminga: “Life from space: A history of panspermia.” Vistas in Astronomy, 1982, Volume 26, p. 69.
16. Lord Kelvin (William Thomson): “On the Origin of Life” Excerpt. Report of the Forty-First Meeting of the British Association for the Advancement of Science, 1871, p. lxxxiv-cv. Available at http://zapatopi.net/kelvin/papers/on_the_origin_of_life.html; accessed August 7, 2007.
17. Harmke Kamminga: “Life from space: A history of panspermia.” Vistas in Astronomy, 1982, Volume 26, p. 70.
18. Ferdinand Cohn: Bacteria, The Smallest Living Things. (Translated by Charles S. Dolley), John Hopkins Press, 1939, p. 15.
19. Thomas Brock: Robert Koch: A Life in Medicine and Bacteriology. ASM Press, 1999, p. 42.
20. Ferdinand Cohn: Bacteria, The Smallest Living Things. (Translated by Charles S. Dolley), John Hopkins Press, 1939 (originally published in 1881), pp. 35-37. Harmke Kamminga: “Life from space: A history of panspermia.” Vistas in Astronomy, 1982, Volume 26, p. 73.
21. Fred Hoyle and Nalin Chandra Wickramasinghe: The Theory of Cosmic Grains. Kluwer Academic Publishers, 1991, pp. 170.
22. Fred Hoyle and Chandra Wickramasinghe: Our Place in the Cosmos, 1993. J.M. Dent, pp. 64-65.
23. Louis Pasteur and Joseph Lister: Germ Theory and its Applications to Medicine & On the Antiseptic Principle of the Practice of Surgery, 1996, Prometheus Books.
24. Douglas Hume: Bechcamp or Pasteur: A Lost Chapter in the History of Biology, 1932. Kessinger’s Publishing.
25. Svante Arrhenius: Worlds in the Making, 1908. (Translated by H. Borns). Harper & Brothers, p. 217.
26. Ibid, p. 214.
27. Harmke Kamminga: “Life from space: A history of panspermia.” Vistas in Astronomy, 1982, Volume 26, p. 80.
28. Bengt Stromgren: “On the development of astrophysics during the last half century”. In J.A. Hynek (Ed.): Astrophysics: A Topical Symposium Commemorating the Fiftieth Anniversary of the Yerkes Observatory and a Half Century of Progress in Astrophysics. McGraw-Hill, 1951, p. 1.