In existographies, Ernst Haeckel (1834-1919) (IQ:180|#202) (Murray 4000:16|B) (FA:74) (GPhE:11) [CR:127] was a German physician, turned Goethe-promoting, Darwin-promoting zoologist, an "unabashed atheist" (Brix, 1992), noted, in hmolscience, for, among things, his "physico-chemical monism" philosophical conception "of the world" (1892) as he put things.
In 1862, Haeckel, in his The Natural History of Creation, employed Darwin to state that the divide between inanimate and animate does not exist.
In 1899, Haeckel, in his The Riddle of the Universe, argues for a very ripe logic; in overview, the following: 
“The fundamental unit of affinity in the whole of nature, from the simplest chemical process to the most complicated love story, [as] was recognized by Empedocles [and] Goethe, [can be] reduced, on logical analysis, to matter (space filling substance) and energy (moving force), [which] are but two inseparable attributes of one underlying substance.”
Herein, among other things, aside from a few difficulties on theory, e.g. reliance on "ether" theory, Haeckel coins the term anthropism, discusses Goethe's Elective Affinities, applies thermodynamics logic to questions of existence, such in the borderland where science begins to overtake religion, e.g. his conception of ‘soul snow’, the nature as to how chemical affinity applied to human passions, among others. 
In 1905, Oliver Lodge penned Life and Matter: a Criticism of Professor Haeckel’s Riddle of the Universe, wherein he attempted to do battle with Haeckel and his Goethe-based monism, by attempting to "ride the fence" with an ontic opening like "incomplete nature" model of physics-based "mechanism and morals".
Haeckel, in his The Riddle of the Universe (pg. 157), via his elaborate soul plasm to cell soul to nervous system theory of soul evolution, came to the conclusion that plants have souls; some of which he comments on as follows:
“The plant-soul (phytopsyche) is, in our view, the summary of the entire psychic activity of the tissueforming, multicellular plant (the metaphyton, as distinct from the unicellular protophyton); it is, however, the subject of the most diverse opinions even at the present day. It was once customary to draw an essential distinction between the plant and the animal, on the ground that the latter had a "soul" and the plant had none. However, an unprejudiced comparison of the irritability and movements of various higher plants and lower animals convinced many observers, even at the beginning of the century, that there must be a "soul" on both sides. At a later date Fechner, Leitgeb (Ѻ) , and others strongly contended for the plant-soul. But a profounder knowledge of the subject was obtained when the similarity of the elementary structure of the plant and of the animal was proved by the cellular theory, and especially when the similarity of conduct of the active, living protoplasm in both was shown in the plasma theory of Max Schultze (1859).”
Here, we might compare Scottish moral philosopher Francis Macnab’s 1818 assertion that plants are neither alive nor dead, an intermediate view to the defunct theory of life, resulting from his attempt to reconcile religious theory (Biblical views) with modern science (fossil evidence, Copernican system, law of gravitation, atomic theory, chemistry, etc.). This nearly begs the nearly similar question: ‘do rocks have souls?’, a query people seem to be have reasoned answers on, depending on if one is a Raramuri Indian (Ѻ), Aristotle (Ѻ), the Ik (Ѻ), African tribesmen of Uganda, or general Yahoo Answers (Ѻ)(Ѻ)(Ѻ) variety, etc.
Law of substance
Haeckel frequently refers to a "law of substance", e.g. in discussions of his theory of a non-supernatural soul, by which he means, as stated in his The Riddle of the Universe (pg. 78), the "formulation of Robert Mayer [conservation of energy] and Helmholtz [conservation of force]". This, to note, seems to have, in its unstated meaning, the "all substances are reducible to energy" theories of Wilhelm Ostwald as conceptualized in his pre-1920s energetics.
Denial | Beliefs
See main: Atheism types by denial and beliefHaeckel, according to James Birx (1992), believed Darwin (believed: evolution) and denied seven main things, namely he rejected: god (was an “unabashed atheist”), vitalism, teleology, mysticism, anthropocentrism, anthropomorphism, and western dualism, of two varieties: (a) that matter and spirit were two separate aspects of existence and (b) that mind-body dualism was unsound. 
Origin of life | Plastidule theory
See main: Plastidue theoryHaeckelm, after reading Darwin in 1860, first began to lecture on the origin of species in 1862.
In 1866, Haeckel, in his General Morphology (Generelle Morphologie), introduced the terms: ‘Plastide’, ‘Plasson’, and ‘Plastidul’, from the Greek plastikos, from plassein to mold or form, as his designation of the first single-celled organisms, creatures of the lowest morphological order of ‘individual’, as his solution to the origin of life problem (which Darwin had patched, in 1871, using a warm pond model).
The creatures or entities themselves Haeckel called ‘Plastiden’, which he considered to be the building blocks from which all life is made, were hypothesized to be filled with a homogeneous substance called ‘Plasson’, which in turn was composed of molecules called ‘Plastidule’ (plural) or ‘Plastidul’ (singular), which were special types of molecules that had the ability to reproduce through a sort of molecular ‘memory’; a conception vaguely said to be similar to or precursory in conception to the way in which the modern view of chromosomes and DNA (molecules) facilitate or rather transmit traits in the process of reproduction. 
The term ‘plastidule’ tends to be rendered in the modern as the "first living molecules"—although this may not exactly be a correct translation, as Haeckel seems to have held a monism viewpoint in his mind.
|A circa 1910 photo (Ѻ) of Haeckel in his studio.|
In his November 1875 lecture “On the Procreation of Life-Particles or Perigenesis of Plastidule” (“Über die Wellenzeugung der Lebensteilchen oder die Perigenesis der Plastidule”), wherein "perigenesis", supposedly, means: “inheritance transmission via a type of growth force possessed by one generation to another”, Haeckel elaborated on his plastidule theory to explain generational transmission. He opened to the straight-forward view that: 
“Every atom has an inherent amount of force and is animated in this sense.”
“Jedes Atom besitzt eine inharente Summe von Kraft und ist in diesem Sinne beseelt.”
He then went on to embraced Goethe's conception of elective affinities, anthropomorphizing atoms using what seems to be the extrapolate down approach: 
“Without the adoption of an 'atomic soul' the most common and general phenomena of chemistry are inexplicable. Pleasure and pain, desire and aversion, attraction and repulsion must be all atomic-masses together, the movements of the atoms, which must take place in formation and release of each chemical compound, are only explicable only if we instill in them sensation and volition. What else is due to basically the generally accepted chemical study of the elective affinity of the body than on the unconscious assumption that in fact the attracting and repelling atoms are inspired by certain tendencies, and that they, these feelings or impulses, following too have the will and the ability to move towards each other and away from each other? What Goethe transmits in his 'Elective Affinities' on the highest composite soul life of man, which has full truth.”
“Ohne die Annahme einer ‘Atomseele’ sind die gewöhnlichsten und allgemeinsten Erscheinungen der Chemie unerklärlich. Lust und Unlust, Begierde und Abneigung, Anziehung und Abstoßung müssen allen Massen-Atomen gemeinsam sein; den die Bewegungen der Atome, die bei Bildung und Auflösung einer jeden chemischen Verbindung stattfinden müssen, sind nur erklärbar, wenn wir ihnen Empfindung und Willen beilegen. Worauf anders beruht den im Grunde die allgemein angenommene chemische Lehre von der Wahlverwandtschaft der Körper, als auf der unbewussten Voraussetzung, dass in der Tat die sich anziehenden und abstoßenden Atome von bestimmten Neigungen beseelt sind, und dass sie, diesen Empfindungen oder Trieben folgend, auch den Willen und die Fähigkeit besitzen, sich zu einander hin und von einander fort zu bewegen? Was Goethe in seinen ‘Wahlverwandtschaften’ über das höchst zusammengesetzte Seelenleben des Menschen überträgt, das besitzt volle Wahrheit …”
Here, to note, in Haeckel's comment on the "will and the ability of atoms to move towards and away from each other", we are reminded of German philosopher and Goethean protege Arthur Schopenhauer's 1818-1844 elective affinity theory of the will of atoms (will to power). It would not be until the development of exchange force theory that this "ability of atoms and molecules to move towards and away from each other" would be reconciled or rather upgraded, in the 1930s, to the model that atoms and molecules of themselves do not have the ability to towards or away from each other, but rather that the transmittal or exchange of field particles is what produced the motion or as it is sometimes explained changes the behavior of the bound state entity ("Massen-Atomen" or atomic mass, as Haeckel seems to define things) that emits or receives the field particle. 
In a 26 May 1876 letter to Haeckel, Bartholomaus von Carneri (1821-1909), in commentary on one of Haeckel's recent publications, stated: 
“I was full of jubilation at the clarity with which you pass from chemical elective affinity one side into the realm of life and on the other into the realm of mechanics; and I am more than ever convinced that in its unremitting progress, science will offer explanations of all the more important points that do not stand in contradiction to the requirements of an austere philosophy, or to put it more correctly, that it makes it possible for an austere philosophy to progress without losing contact with the ground under it's feet.”
Memory of atoms
Haeckel commented, at some point, that he got his idea of the “memory of atoms” (whatever he means by this?) from Goethe’s 1809 Elective Affinities. 
Elective affinity | Review
The following is Haeckel's 1899 summary of Goethe’s novella-version theory of human chemical affinities: 
“Goethe, in his classical romance, Elective Affinities, compares the relations of pairs of loves with the phenomenon of the same name in the formation of chemical combinations. The irresistible passion that draws Edward to the sympathetic Ottilie, or Paris to Helen, and leaps over all bounds of reason and morality, is the same powerful unconscious attractive force which impels the living spermatozoon to force an entrance into the ovum in the fertilization of the egg of the animal or plant—the same impetuous movement which unities the two atoms of the hydrogen to one atom of the oxygen for the formation of the a molecule of water. This fundamental unity of affinity in the whole of nature was recognized by the great Greek scientist Empedocles in the fifth century BC in his theory of the love and hatred of the elements.”
|Haeckel's 1874 "evolution of man" diagram, showing: 1 Amoeba; 1a Asexual reproduction (amoeba dividing); 2 Sexual reproduction (cell with spore); 3 Multi-cellular organism (early embryonic stage); 4 Muliticellular organism with three germ layers (blastula); 5 Organism with primitive mouth (gastrula); 6 Planaria; 7 Worm (leech); 8 Primitive chordate (tunicate larva); 8a Adult tunicate; 9 Lancelet; 10 Jawless fish (lamprey); 11 Cartilaginous fishes (shark); 12 Australian lungfish; 13 South American lungfish; 14 Aquatic reptile (plesiosaur); 15 Early amphibian (labytinthodont); 16 Modern amphibian (newt); 17 Reptile (iguana); 18 Monotreme (platypus); 19 Marsupial (kangaroo); 20 Prosimian (lemur); 21 Monkey (langur); 22 Ape (orangutan); 23 Ape-man (Pithecanthropus); 24 Modern human (a Papuan).|
Elective affinity | Psychological chemotropism
In 1893 to 1903, Haeckel, in his love letters, mentions the concept of elective affinity at least three times in respect to his own romantic relationships. In one letter to a Franziska von Altenhausen, Haeckel defines elective affinity as a strange psychological chemotropism: 
“… seductive women—why should I, despite all scruples and obstacles, cast myself into the dust before you? Dearst Franziska, herein lies the enigma of ‘elective affinity’, of that strange psychological ‘chemotropism’, of whose power I have spoken repeatedly in my books—little dreaming that I myself should fall a victim to it in my old age!”
Haeckel was a rather unique soul theorist, as soul theorists go, attributed "souls", eventually, to all entities: organism, cells, tissues, nerves, crystals, atoms, and light-bearing ether.  In his 1874 Anthropogeny (The Origin of Humans), Haeckel began to use a mechanical explanation of nature, albeit in panpsychism sort of way, so to speak, that energy and soul are synonymously denoted by the term “energy” (or possibly kraft). 
One peculiar example of his material soul theory is his temperature-based "soul snow" conception, one of the more forced religion-melded-to-science contrivances known to science, thermodynamics or rather religious thermodynamics in particular.
His last book, the 1917 Crystal Souls: Studies on Inorganic Life, was devoted to extolling his views on souls in liquid crystals. 
Descent of man | 1874
In 1874, Haeckel gave what seems to be one of the first "evolution of man", diagrams (adjacent), from amoeba to modern human, "great chain of being" like diagram, illustrated by modern and fossil species:  Of note here we see the prototype of a the organic | inorganic origin of life divide forming, namely the amoeba (first form of life) | inorganic matter (parts of amoeba) conceptual demarcation. A fuller reading of Haeckel's works, however, are in order here, to see what he conceptualized before or at step one (amoeba)?
Haeckel grew up listening to his mother recite Schiller’s poetry and discussing Goethe’s nature philosophy with his father.  Haeckel, in his General Morphology (1866), commented in regard to his earlier educational years:
“In our most impressionable years, we are given very defective, perverse, and often really mischievous instruction, by which we are filled with absurd errors, instead of natural truths.”
In 1858, Haeckel completed his MD in Berlin in under Johannes Muller, among others, in biological sciences, wherein he adopted the Helmholtz school of thought, that of the mechanical theory of life over that of the vitalism theory of life. Haeckel then completed a DSc in biology and PhD in zoology, the latter at the University of Jena, where at he became a professor of comparative anatomy, remaining there for 47 years, until 1909.
Quotes | By
The following are noted quotes:
“We thus arrive at the extremely important conviction that all natural bodies which are known to us, are equally animated, that the distinction which has been made between animate and inanimate bodies does not exist.”— Ernst Haeckel (1868), The History of Creation, key-citation from above 
“Darwin is the ‘new Newton’ who has explained the origin of organisms strictly via mechanical causes.”— Ernst Haeckel (1868), History of Creation 
“Judeo-Christian beliefs are ‘those dogmatic doctrines of anthropism’.”— Ernst Haeckel (c.1868) 
“Wolfgang Goethe: Germany’s greatest genius.”— Ernst Haeckel (1892), Monism 
“The immortal merit of establishing the doctrine on an empirical basis, and pointing out its world-wide application, belongs to the great scientist Charles Darwin; he it was who, in 1859, supplied a solid foundation foi the theory of descent, which the able French naturalist Jean Lamarck had already sketched in its broad outlines in 1809, and the fundamental idea of which had been almost prophetically enunciated in 1799 by Germany's greatest poet and thinker, Wolfgang Goethe. These three distinguished naturalists shine as three stars of the first magnitude amid all the great men of the century.”— Ernst Haeckel (1899), The Riddle of the Universe (pgs. 4-5)
“The fundamental unit of affinity in the whole of nature, from the simplest chemical process to the most complicated love story, [as] was recognized by Empedocles [and] Goethe, [can be] reduced, on logical analysis, to matter (space filling substance) and energy (moving force), [which] are but two inseparable attributes of one underlying substance.”— Ernest Haeckel (1899), The Riddle of the Universe (pg. #)
“During the Middle Ages—especially during the domination of the papacy—scientific work in this direction entirely ceased. The torture and the stake of the Inquisition insured that an unconditional belief in the Hebrew mythology should be the final answer to all the questions of creation.”— Ernst Haeckel (1899), The Riddle of the Universe (pg. 238)
“As the coach pulled up to Darwin’s ivy-covered country house, shaded by elms, out of the shadows of the vine-covered entrance came the great scientist himself to meet me. He had a tall, worthy form with the broad shoulders of Atlas, who carries a world of thought. He had a Jupiter-like forehead, high and broadly domed, similar to Goethe’s, and with deep furrows from the habit of mental work.”— Ernst Haeckel (c.1900), retrospect on first meeting Charles Darwin 
1. (a) Haeckel, Ernst. (1895–1899) Die Welträthsel ("world-riddle"); also spelled Die Welträtsel. Jena: Publisher.
(b) Haeckel, Ernst. (1899). The Riddle of the Universe: at the Close of the Nineteenth Century (translator: Joseph McCabe) (6: The Nature of the Soul, pgs. 88-108; §11: Immortality of the Soul, pgs. 188-210; §12: The Law of Substance, pgs. 211-32; soul-snow, pg. 201; affinity, pg. 224; aggregate quote, pgs. 216+224). Harper & Brother, 1900.
(c) The Riddle of the Universe – Wikipedia.
2. Ball, Philip. (2011). Unnatural: the Heretical Idea of Making People (pg. 129). Vintage Books.
3. Botar, Oliver A.I. and Vunsche, Isabel. (2011). Biocentrism and Modernism (pgs. 85-88). Ashgate Publishing.
4. Nordenskiold, Erik. (1928). The History of Biology: a Survey (pgs. 519-20). Taylor & Francis.
5. (a) Jacobensen, Eric P. (2005). From Cosmology to Ecology: the Monist World-View in Germany from 1770 to 1930 (pgs. 110-11). Peter Lang.
(b) Haeckel, Ernst. (1875). “On the Procreation of Life-Particles or the Generation of Waves in Protoplasm (“Über die Wellenzeugung der Lebensteilchen oder die Perigenesis der Plastidue”), delivered at Jena on November 19, 1875, before the Medical-Scientific Society (Medizinisch-naturwissenschaftliche Gesellschaft); In GV, vol. 2 (1902): 60.
6. Thims, Libb. (2007). Human Chemistry (Volume One) (ch. 7: Bound State Interactions, pgs. 183-211). Morrisville, NC: LuLu.
7. Jacobensen, Eric P. (2005). From Cosmology to Ecology: the Monist World-View in Germany from 1770 to 1930 (pg. 106). Peter Lang.
8. Haeckel, Ernst. (1930). The Love Letters of Ernst Haeckel: Written Between 1898 and 1903 (editor: Johannes Werner) (elective affinity, pgs. 101, 212, 260). Methuen.
9. Di Gregorio, Mario A. (2005). From Here to Eternity: Ernst Haeckel and Scientific Faith (pg. 389). Vandenhoeck & Ruprecht.
10. Haeckel, Ernst. (1874). Anthropogenie oder Entwicklungsgeschichte des Menschen (image). Publisher.
11. (a) Haeckel, Ernst. (1868). The History of Creation: The Development of the Earth and Its Inhabitants by the Action of Natural Causes, Volume One (Natürliche Schöpfungs-geschichte) (translator: E. Ray Lankester) (pg. 23). Publisher.
(b) Haeckel, Ernst. (1868). The History of Creation: The Development of the Earth and Its Inhabitants by the Action of Natural Causes, Volume Two (Natürliche Schöpfungs-geschichte) (translator: E. Ray Lankester). Publisher.
(c) Bray, Henry T. (1910). The Living Universe (pg. 204). Truro Publishing Co., 1920.
(d) Wiker, Benjamin. (2009). Moral Darwinism: How We Became Hedonists (pgs. 257-58). InterVarsity Press.
12. Haeckel, Ernst. (1930). The Love Letters of Ernst Haeckel: Written Between 1898 and 1903 (editor: Johannes Werner) (elective affinity, pgs. 101, 212, 260). Methuen.
13. (a) Haeckel, Ernst. (1868). (History of Creation) Natürliche Schöpfungsgeschichte (pg. 95). Reimer, 1889.
(b) Cornell, John F. (1986). “Newton of the Grassblade? Darwin and the Problem of Organic Teleology” (Ѻ), Isis, 77:405-21.
(c) McGrath, Alister E. (2015). Dawkins’ God: from the Selfish Gene to the God Delusion (pg. 114). John Wiley & Sons.
14. (a) Birx, H. James. (1992). “Introduction”, in: The Riddle of the Universe (by: Ernst Haeckel, translator: Joseph McCabe) (pgs. ix-xiv). Prometheus Books, 1992.
(b) H. James Birx – Wikipedia.
16. Haeckel, Ernst. (1892). Monism as Connecting Religion and Science: the Confessions of Faith of a Man of Science, Informal Address/Lecture delivered extemporaneously, at Altenburg, on the 75th anniversary of the Naturforschende Gesellschaft des Osterlandes, Oct 9; (translator: J. Gilchrist) (affinity, 4+ pgs; Goethe, 6+ pgs; greatest genius, pg. 84). Adam and Charles Black, 1894.
● Haeckel, Ernst. (1862). The History of Creation, Or, the Development of the Earth and Its Inhabitants by the Action of Natural Causes: A Popular Exposition of the Doctrine of Evolution in General, and of that of Darwin, Goethe, and Lamarck in Particular, Volume One (translator: E. Ray Lankkester). Trübner & Company Limited, 1892.
● Haeckel, Ernst. (1862). The History of Creation, Or, the Development of the Earth and Its Inhabitants by the Action of Natural Causes: A Popular Exposition of the Doctrine of Evolution in General, and of that of Darwin, Goethe, and Lamarck in Particular, Volume Two (translator: E. Ray Lankkester). D. Appleton and Co., 1884.
● Haeckel, Ernst. (1866). General Morphology of Organisms, Volumes 1-2. Publisher.
● Haeckel, Ernst. (1874). Anthropology. Publisher.
● Haeckel, Ernst. (1877). Freedom in Science and Teaching. Publisher.
● Haeckel, Ernst. (1892). “Our Monism: the Principles of a Consistent, Unitary World-View” (pdf), The Monist, 4(2):481-86, Jul.
● Haeckel, Ernst. (1899). The Riddle of the Universe: at the Close of the Nineteenth Century. Publisher.
● Haeckel, Ernst. (1909). “Charles Darwin as an Anthropologist”, Publication.
● Haeckel, Ernst. (1916). Eternity. Publisher.
● Ernst Haeckel – Wikipedia.
● Die Perigenesis der Plastidule – Britannica.