Haeckel, the " indomitable worker ", quickly produced his History of Creation.

Influenced by thinkers like Darwin and zoologist Ernst Heinrich Haeckel, he published several papers.

During these times, he worked several times at facilities located by the sea: Heligoland alongside Haeckel in 1865, Hamburg in 1866, Millport, Scotland with David Robertson in 1867 and 1868 and moved to Messina, Italy, during the winter of 1868 together with his friend and colleague Nicholas Miklouho-Maclay to work on the marine life of the Straits of Messina.

In contrast to most of Darwin's supporters, Ernst Haeckel put forward a doctrine of evolutionary polygenism based on the ideas of the linguist and polygenist August Schleicher, in which several different language groups had arisen separately from speechless prehuman Urmenschen, which themselves had evolved from simian ancestors.

late 20th and early 21st century critics, Jonathan Wells and Stephen Jay Gould, have objected to the continued use of Haeckel ’ s embryo drawings in textbooks.

Romanes ' 1892 copy of Ernst Haeckel's allegedly fraudulent embryo drawings ( this version of the figure is often attributed incorrectly to Haeckel ).

Haeckel ’ s embryo drawings are primarily intended to express his idiosyncratic theory of embryonic development, the Biogenetic Law, which in turn assumes ( but is not crucial to ) the evolutionary concept of common descent.

Haeckel ’ s ‘ Biogenetic Law ’ portrays the parallel relationship between an embryo ’ s development and phylogenetic history.

In addressing his embryo drawings to a general audience, Haeckel does not cite any sources, which gives his opponents the freedom to make assumptions regarding the originality of his work.

The debate between Haeckel and His ultimately becomes fueled by the description of an embryo that Wilhelm Krause propels directly into the ongoing feud between Haeckel and His.

Although Haeckel is proven right about the allantois, the utilization of Krause ’ s embryo as justification turns out to be problematic, for the embryo is that of a bird rather than a human.

Ultimately, His goes so far as to accuse Haeckel of “ faking ” his embryo illustrations to make the vertebrate embryos appear more similar than in reality.

Rutimeyer claimed that Haeckel “ had taken to kinds of liberty with established truth .” Rutimeyer claimed that Haeckel presented the same image three consecutive times as the embryo of the dog, the chicken, and the turtle.

Although Rutimeyer did not denounce Haeckel ’ s embryo drawings as fraud, he argued that such drawings are manipulations of public and scientific thought.

Nevertheless, Bischoff ’ s main argument was in reference to Haeckel ’ s drawings of human embryos, for Haeckel is later accused of miscopying the dog embryo from him.

Throughout Haeckel ’ s time, criticism of his embryo drawings was often due in part to his critics ' belief in his representations of embryological development as “ crude schemata .” In this way, Haeckel specifically selected relevant features to portray in his drawings.

In 1877, Rudolf Virchow ( 1821 – 1902 ), once an inspiration to Haeckel at Würzburg, proclaimed that Haeckel ’ s embryo drawings represent mere hypotheses.

Romanes's 1892 copy of Ernst Haeckel's controversial embryo drawings ( this version of the figure is often attributed incorrectly to Haeckel ).

For example, Haeckel proposed that the pharyngeal grooves between the pharyngeal arches in the neck of the human embryo resembled gill slits of fish, thus representing an adult " fishlike " developmental stage as well as signifying a fishlike ancestor.

Ernst Haeckel ( 1866 ), in his endeavour to produce a synthesis of Darwin's theory with Lamarckism and Naturphilosophie, proposed that " ontogeny recapitulates phylogeny ," that is, the development of the embryo of every species ( ontogeny ) fully repeats the evolutionary development of that species ( phylogeny ), in Geoffroy's linear model rather than Darwin's idea of branching evolution.

The presence of gill-like slits in the neck of the developing human embryo famously led Ernst Haeckel to postulate that " ontogeny recapitulates phylogeny "; this hypothesis, while false, contains elements of truth, as explored by Stephen Jay Gould in Ontogeny and Phylogeny.

On the other hand, Michael K. Richardson, Professor of Evolutionary Developmental Zoology, Leiden University, while recognizing that some criticisms of the drawings are legitimate ( indeed, it was he and his co-workers who began the modern criticisms in 1998 ), has supported the drawings as teaching aids, and has said that " on a fundamental level, Haeckel was correct "

In his drawings, Haeckel cites the notochord, pharyngeal arches and clefts, pronephros and neural tube as palingenetic features.

Haeckel was not the only one to create a series of drawings representing embryonic development.

Michael Richardson and his colleagues in a July 1997 issue of Anatomy and Embryology, demonstrated that Haeckel fudged his drawings in order to exaggerate the similarity of the phylotypic stage.

In a March 2000 issue of Natural History, Stephen Jay Gould argued that Haeckel “ exaggerated the similarities by idealizations and omissions .” As well, Gould argued that Haeckel ’ s drawings are simply inaccurate and falsified.

Some version of Haeckel ’ s drawings can be found in many modern biology textbooks in discussions of the history of embryology, with clarification that these are no longer considered valid.

" Moreover, " vertebrate embryos, for most of the longest period of middevelopment, do look remarkably alike, pretty much, but not exactly, as Haeckel figured them in some of his drawings "( emphasis in original ).

While often seen as rejecting Darwin's theory of branching evolution for a more linear Lamarckian " biogenic law " of progressive evolution, this is not accurate: Haeckel used the Lamarckian picture to describe the ontogenic and phylogenic history of the individual species, but agreed with Darwin about the branching nature of all species from one, or a few, original ancestors.

Yet, in our times, scientific hylozoism – whether modified, or keeping the trend to make all beings conform to some uniform pattern, to which the concept was adhered in modernity by Herbert Spencer, Hermann Lotze, and Ernst Heinrich Haeckel – was often called upon as a protest against a mechanicistic view of the world.

Although the early embryos of different species exhibit similarities, Haeckel apparently exaggerated these similarities in support of his Recapitulation theory, sometimes known as the Biogenetic Law or " Ontogeny recapitulates phylogeny ".

Haeckel ’ s opponents believe that he de-emphasizes the differences between early embryonic stages in order to make the similarities between embryos of different species more pronounced.

Haeckel advanced a version of the earlier " recapitulation theory ", previously set out by Étienne Serres in the 1820s and supported by followers of Geoffroy including Robert Edmond Grant, which proposed a link between ontogeny ( development of form ) and phylogeny ( evolutionary descent ), summed up by Haeckel in the phrase " ontogeny recapitulates phylogeny ".

The underlying debate between Haeckel and His derives from differing viewpoints regarding the similarity or dissimilarity of vertebrate embryos.

Haeckel ’ s opponents found his methods problematic because such simplification eliminates certain structures that differentiate between higher and lower vertebrates.

In determining the relationships between " phylogenetic linkages " and " evolutionary laws of form ," both Gegenbaur and Haeckel relied on a method of comparison.

Early attempts to identify relationships between major groups were made in the 19th century by Ernst Haeckel, and by comparative anatomists such as Thomas Henry Huxley and E. Ray Lankester.

Nevertheless, his chief interest was in human evolution, influenced by Ernst Haeckel, who reasoned that there must be intermediate species between apes and human.

Extensive correspondence exists between Müller and Darwin, and Müller also corresponded with Hermann Müller, Alexander Agassiz, Ernst Krause and Ernst Haeckel.

Haeckel ’ s illustrations show vertebrate embryos at different stages of development, which exhibit embryonic resemblance as support for evolution, recapitulation as evidence of the Biogenetic Law, and phenotypic divergence as evidence of von Baer ’ s laws.

The series of twenty-four embryos from the early editions of Haeckel ’ s Anthropogenie remain the most famous.

Haeckel proposes that all classes of vertebrates pass through an evolutionarily conserved “ phylotypic ” stage of development, a period of reduced phenotypic diversity among higher embryos.

His, in opposition to Haeckel, seeks to take human embryos out of the hands of Darwinist proponents.

In response to Haeckel ’ s evolutionary claim that all vertebrates are essentially identical in the first month of embryonic life as proof of common descent, His responds by insisting that a more skilled observer would recognize even sooner that early embryos can be distinguished.

His also accuses Haeckel of creating early human embryos that he conjured in his imagination rather than obtained through empirical observation.

In fact, ontogeny evolves – the phylogenetic history of a species cannot be read directly from its ontogeny, as Haeckel thought would be possible, but characters from ontogeny can be ( and have been ) used as data for phylogenetic analyses ; the more closely related two species are, the more apomorphies their embryos share.

One issue they highlighted was Wells ' accusation that Haeckel forged images of embryos that are allegedly still in biology books.