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.

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.

Similarities can be seen along the first two rows ; the appearance of specialized characters in each species can be seen in the columns and a diagonal interpretation leads one to Haeckel ’ s idea of recapitulation.

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.

Ernst Haeckel, along with Karl von Baer and Wilhelm His, are primarily influential in forming the preliminary foundations of ‘ phylogenetic embryology ’ based on principles of evolution.

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

The term, ‘ recapitulation ,’ has come to embody Haeckel ’ s Biogenetic Law, for embryonic development is a recapitulation of evolution.

Haeckel portrays a concrete demonstration of his Biogenetic Law through his ‘ Gastrea ’ theory, in which he argues that the early cup-shaped gastrula stage of development is a universal feature of multi-celled animals.

As a response to Haeckel ’ s theory of recapitulation, von Baer enunciates his most notorious laws of development.

Wilhelm His was one of Haeckel ’ s most authoritative and primary opponents advocating physiological embryology.

His ’ depiction of embryological development strongly differs from Haeckel ’ s depiction, for His argues that the phylogenetic explanation of ontogenetic events is unnecessary.

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.

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.

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.

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

As a pioneer in mammalian embryology, he was one of Haeckel ’ s strongest critics.

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.

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.

For example, Haeckel described and named hypothetical ancestral microorganisms that have never been found.

Haeckel postulated that evidence of human evolution would be found in the Dutch East Indies ( now Indonesia ), and described these theoretical remains in great detail.

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.

As a result of his letters and, no doubt, personal conversations, Huxley and Haeckel were convinced that, at the time he wrote Principles, he believed new species had arisen by natural methods.

Although Haeckel stressed comparative embryology and Gegenbaur promoted the comparison of adult structures, both believed that the two methods could work in conjunction to produce the goal of evolutionary morphology.

This contention may look convincing at first glance inasmuch as On the Origin of Species is full of observations and proposed mechanisms that clearly fit within the boundaries of modern ecology ( e. g. the cat-to-clover chain – an ecological cascade ) and because the term ecology was coined in 1866 by a strong proponent of Darwinism, Ernst Haeckel.

Ernst Haeckel claimed that Negroes have stronger and more freely movable toes than any other race which is evidence that Negroes are connected to apes because when apes stop climbing in trees they hold on to the trees with their toes, Haeckel compared Negroes to “ four-handed ” apes.

Despite the numerous oppositions, Haeckel has influenced many disciplines in science in his drive to integrate such disciplines of taxonomy and embryology into the Darwinian framework and to investigate phylogenetic reconstruction through his Biogenetic Law.

Historians write that most such political and economic commentators had only a superficial understanding of Darwin's scientific theory, and were as strongly influenced by other concepts about social progress and evolution, such as the Lamarckian ideas of Spencer and Haeckel, as they were by Darwin's work.

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.

Indeed, it was the question of where to put such " unclassifiable " creatures that prompted Ernst Haeckel to add a third kingdom to the Animale and Vegetabile of Linnaeus: the Kingdom Protista.

Müller mentored such distinguished scientists and physiologists as Hermann von Helmholtz, Emil du Bois-Reymond, Theodor Schwann, Friedrich Gustav Jakob Henle, Carl Ludwig and Ernst Haeckel.

Early twentieth-century biologists like Ernst Haeckel viewed embryology as a recapitulation of evolution, which implies a kind of organising memory, and a few modern biologists, such as Rupert Sheldrake, influenced by Jungian ideas and by vitalism, have posited organising fields of life consisting of memories and drives.

In what was to be the last decade of his life, he penned works such as Parsifal Unveiled, which details the esoteric symbolism of the Wagner opera, and Gnostic Anthropology in which he heavily criticizes the theories of Darwin, Haeckel, " and their followers ".

Ehret was a proponent of the emerging back-to-nature renaissance in Germany and Switzerland during the latter part of the 19th century, which was inspired by writers such as Meister Eckhart, Christoph Wilhelm Hufeland, Nietzche, Goethe, Herman Hesse, Ernst Haeckel and Eduard Baltzer as well as the healing traditions of Roman and Greek philosophers such as Paracelsus, Empedocles, Seneca, Plutarch, Porphyry, Galen, Hippocrates, Socrates, Pythagoras, Plato and Aristotle.

Furthermore, Haeckel even proposed theoretical life-forms to accommodate certain stages in embryogenesis.

Haeckel argues that certain features in embryonic development are conserved and palingenetic, while others are caenogenetic.

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 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.

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

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

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.

Haeckel produced several embryo drawings that often overemphasized similarities between embryos of related species.

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.