When Haeckel was a student in the 1850s he showed great interest in
embryology, attending the rather unpopular lectures twice and in his notes sketched the visual aids: textbooks had few illustrations, and large format plates were used to show students how to see the tiny forms under a reflecting microscope, with the translucent tissues seen against a black background. Developmental series were used to show stages within a species, but inconsistent views and stages made it even more difficult to compare different species. It was agreed by all European evolutionists that all
vertebrates looked very similar at an early stage, in what was thought of as a common ideal type, but there was a continuing debate from the 1820s between the Romantic
recapitulation theory that human embryos developed through stages of the forms of all the major groups of adult animals, literally manifesting a sequence of organisms on a
linear chain of being, and
Karl Ernst von Baer's opposing view, stated in
von Baer's laws of embryology, that the early general forms diverged into four major groups of specialised forms without ever resembling the adult of another species, showing affinity to an
archetype but no relation to other types or any
transmutation of species. By the time Haeckel was teaching he was able to use a textbook with woodcut illustrations written by his own teacher
Albert von Kölliker, which purported to explain human development while also using other mammalian embryos to claim a coherent sequence. Despite the significance to ideas of transformism, this was not really polite enough for the new popular science writing, and was a matter for medical institutions and for experts who could make their own comparisons.
Darwin, Naturphilosophie and Lamarck Darwin's
On the Origin of Species, which made a powerful impression on Haeckel when he read it in 1864, was very cautious about the possibility of ever reconstructing the history of life, but did include a section reinterpreting von Baer's embryology and revolutionising the field of study, concluding that "Embryology rises greatly in interest, when we thus look at the embryo as a picture, more or less obscured, of the common parent-form of each great class of animals." It mentioned von Baer's 1828 anecdote (misattributing it to
Louis Agassiz) that at an early stage embryos were so similar that it could be impossible to tell whether an unlabelled specimen was of a mammal, a bird, or of a reptile, and Darwin's own research using embryonic stages of
barnacles to show that they are
crustaceans, while cautioning against the idea that one organism or embryonic stage is "higher" or "lower", or more or less evolved. Haeckel disregarded such caution, and in a year wrote his massive and ambitious
Generelle Morphologie, published in 1866, presenting a revolutionary new synthesis of Darwin's ideas with the German tradition of
Naturphilosophie going back to
Goethe and with the progressive evolutionism of
Lamarck in what he called
Darwinismus. He used
morphology to reconstruct the
evolutionary history of life, in the absence of fossil evidence using embryology as evidence of ancestral relationships. He invented new terms, including
ontogeny and
phylogeny, to present his evolutionised recapitulation theory that "ontogeny recapitulated phylogeny". The two massive volumes sold poorly, and were heavy going: with his limited understanding of German, Darwin found them impossible to read. Haeckel's publisher turned down a proposal for a "strictly scholarly and objective" second edition.
Controversy Later in 1874, Haeckel's simplified embryology textbook
Anthropogenie made the subject into a battleground over Darwinism aligned with Bismarck's
Kulturkampf ("culture struggle") against the Catholic Church. Haeckel took particular care over the illustrations, changing to the leading zoological publisher Wilhelm Engelmann of Leipzig and obtaining from them use of illustrations from their other textbooks as well as preparing his own drawings including a dramatic double page illustration showing "early", "somewhat later" and "still later" stages of 8 different vertebrates. Though Haeckel's views had attracted continuing controversy, there had been little dispute about the embryos and he had many expert supporters, but
Wilhelm His revived the earlier criticisms and introduced new attacks on the 1874 illustrations. Expert anatomists were critical of Haeckel's work. Catholic priests also opposed Haeckel's views. Specifically, Haeckel's critics accused him of manipulating the embryo drawings to make the early stages of different species look more similar. They claimed that the drawings of four-week dog and human embryos had been copied without attribution from other sources, and changed by expanding dog's head and reducing the human head, moving the eye, and exaggerating the size of the human tail. Haeckel himself considered the drawings stylized and illustrative, but his detractors considered them forgery and fraud. Recent analyses (Richardson 1998, Richardson and Keuck 2002) have found that some of the criticisms of Haeckel's embryo drawings were legitimate, but others were unfounded. There were multiple versions of the embryo drawings, and Haeckel rejected the claims of fraud. It was later said that "there is evidence of sleight of hand" on both sides of the feud between Haeckel and
Wilhelm His.
Robert J. Richards, in a paper published in 2008, defends the case for Haeckel, shedding doubt against the fraud accusations based on the material used for comparison with what Haeckel could access at the time. == Assessments of potential influence on Nazism ==