On this date, the German biologist, naturalist, philosopher, physician, and artist Ernst Heinrich Philipp August Haeckel was born at Potsdam. He is probably one of the most contentious evolutionary biologists that ever lived. He abandoned his medical practice after reading Charles Darwin’s The Origin of Species in 1859 and returned to school, studying zoology and anatomy and eventually earning a position as professor in Jena.
Haeckel embraced the pre-Darwinian notion that life formed a series of successively higher forms, with embryos of higher forms “recapitulating” the lower ones. He thought that, over the course of time, evolution of new life forms occurred by the addition of new adult stages to the end of ancestral developmental sequences. Haeckel, who was very good at packaging and promoting his ideas, coined both a name for the process – “the Biogenetic Law” – as well as a catchy motto: “Ontogeny Recapitulates Phylogeny.”
Thus, according to Haeckel, embryonic development was actually a record of evolutionary history. He expressed it this way, as quoted in Russell (1916) :
The organic individual… repeats during the rapid and short course of its individual development the most important of the form changes which its ancestors traversed during the long and slow course of their paleontological evolution…
The human zygote, for instance, was represented by the “adult” stage of the protists; the colonial protists represented the advancement of development to the blastula stage; the gill slit stage of human embryos was represented by adult fish. Haeckel even postulated an extinct organism, Gastraea, a two-layered sac corresponding to the gastrula, which he considered the ancestor of all metazoan species. 
PZ Meyers at Talk.Origins Archive  writes that the Biogenetic Law as conceived by Haeckel says:
…that development (ontogeny) repeats the evolutionary history (phylogeny) of the organism – that if we evolved from a fish that evolved into a reptile that evolved into us, our embryos physically echo that history, passing through a fish-like stage and then into a reptile-like stage.
Haeckel was so convinced of his biogenetic law that he was willing to bend evidence to support it. In 1874, he had claimed that members of all vertebrate classes pass through an identical evolutionarily conserved “phylotypic” stage, which presumably represents the form of their most recent common ancestor. Only later in development would specific differences appear, he said.
In fact, there is a highly conserved embryonic stage among the vertebrate classes; at the late tailbud stage, vertebrate embryos of most all classes possess somites, neural tube, optic anlagen, notochord, and pharyngeal pouches. However, Michael Richardson and his colleagues (1997)  discovered significant differences between groups at this stage. For example, in echidnas, limb buds are already present at the tailbud stage, whereas in other species, these are not seen until significantly later.
But in his illustrations of vertebrate embryos, Haeckel deceptively omitted limb buds at an early stage of the echidna, despite the fact that limb buds do exist then, in order to make his vertebrate embryos look more alike than they do in real life. Haeckel’s motive is clear from the text accompanying his drawings: “There is still no trace of the limbs or ‘extremities’ in this stage of development…”. 
Near the conclusion of the Brass-Haeckel Controversy  of 1908, after months of vociferous and emphatic denial, in the Berliner Volkszeitung published on 29 December 1908, Haeckel apparently admitted he had altered drawings of embryos, as quoted in Haeckel’s Frauds and Forgeries (1915) :
To cut short this unsavory dispute, I begin at once with the contrite confession that a small fraction of my numerous drawings of embryos (perhaps 6 or 8 per cent.) are really, in Dr. Brass’s sense, falsified – all those, namely, for which the present material of observation is so incomplete or insufficient as to compel us, when we come to prepare a continuous chain of the evolutive stages, to fill up the gaps by hypotheses, and to reconstruct the missing-links by comparative syntheses… After this compromising confession of “forgery” I should be obliged to consider myself “condemned and annihilated,” if I had not the consolation of seeing side-by-side with me in the prisoner’s dock hundreds of fellow-culprits, among them many of the most trusted observers and most esteemed biologists. For the great majority of all the figures – morphological, anatomical, histological, and embryological – that are widely circulated and valued in the best text- and handbooks, in biological treatises and journals, would incur in the same degree the charge of “forgery.” All of them are inexact, and are more or less “doctored,” schematised, or “constructed.” Many unessential accessories are left out, in order to render conspicuous what is essential in form and organisation. [ellipsis in original]
The truth is that the development of embryos does not fit into the strict progression that Haeckel claimed, but it has also been shown that ontogeny (development of a fertilized ovum through to maturity) and phylogeny (development of a species over time) are closely related. That is, similar features in embryos of different species often reliably demonstrate that the species share a recent common ancestor. This is nicely summarized by Douglas Theobald online at Talk.Origins Archive :
Interestingly, in 1866 Haeckel created the first evolutionary tree to incorporate all life known at the time.
The ideas of Ernst Haeckel greatly influenced the early history of embryology in the 19th century. Haeckel hypothesized that “Ontogeny Recapitulates Phylogeny”, meaning that during its development an organism passes through stages resembling its adult ancestors. However, Haeckel’s ideas long have been superseded by those of Karl Ernst von Baer, his predecessor. Von Baer suggested that the embryonic stages of an individual should resemble the embryonic stages of other closely related organisms, rather than resembling its adult ancestors. Haeckel’s Biogenetic Law has been discredited since the late 1800’s, and it is not a part of modern (or even not-so-modern) evolutionary theory. Haeckel thought only the final stages of development could be altered appreciably by evolution, but we have known that to be false for nearly a century. All developmental stages can be modified during evolution… [emphasis in original]
Although a strong supporter and defender of evolution (especially against attacks from religious leaders), Haeckel did not share Darwin’s enthusiasm for natural selection as the main mechanism for generating the diversity of the biological world. Instead, he favored a type of Lamarckism.
According to Jean-Baptiste Lamarck (1744-1829), change in the environment causes change in the behavior of individuals; altered behavior leads to greater or lesser use of a given structure or organ. Use would cause the structure to increase in size over many generations, whereas disuse would cause it to shrink or even disappear, because physical characteristics acquired by parents during their lifetimes are passed along to their offspring. The mechanism of Lamarckian evolution is quite different from that proposed by Darwin, although the predicted result is the same: adaptive change in lineages, ultimately driven by environmental change, over long periods of time.
PZ Meyers explains in his essay why Haeckel was completely wrong:
He argued that evolutionary history was literally the driving force behind development, and that the experiences of our ancestors were physically written into our hereditary material. This was a logical extension of his belief in Lamarckian inheritance, or the inheritance of acquired characters. If the activity of an organism can be imprinted on its genetics, then development could just be a synopsis of the activities of the parents and grandparents and ever more remote ancestors. This was an extremely attractive idea to scientists; it’s as if development were a time machine that allowed them to look back into the distant past, just by studying early stages of development.
Unfortunately, it was also completely wrong.
The discoveries that ultimately demolished the underlying premises of the biogenetic law were the principles of genetics and empirical observations of embryos. Lamarckian inheritance simply does not occur… DNA is the agent of heredity, and it is not modified by our ordinary actions – if you should get a tattoo, it is not also written into the chromosomes of your sperm or ova, and there’s no risk that your children will be born with “Mom” etched on their arm. The discovery that Haeckel had taken unforgivable shortcuts with his illustrations was a relatively minor problem for his theory, because the general thrust of his observations (that vertebrate embryos resemble each other strongly) had been independently confirmed. What really scuttled the whole theory was that its foundation was removed.
Much later, Haeckel attempted to develop a comprehensive philosophical system informed by biological and evolutionary findings. This system was to encompass ethics, theology, psychology, and politics. Some authors claim that Haeckel’s work was later appropriated by the Nazis who used it as justification for their racism and nationalism.  Others dispute that claim. Complicating this issue is the fact that, depending on whether you disparage or praise Haeckel, you are often assumed to be either a fundamentalist Christian, opposed to evolution, or an atheist, opposed to morality.
- E. S. Russell. Form and Function (London: John Murray Ltd., 1916) p. 253.
- Ernst Haeckel. Generelle Morphologie der Organismen (Berlin: Georg Reimer, 1867).
- —————— Anthropogenie. Third edition. (Leipzig: W. Engelmann, 1879).
- S.J. Gould. Ontogeny and Phylogeny (Cambridge, MA: Harvard University Press, 1977).
- PZ Meyers. “Wells and Haeckel’s Embryos: A Review of Chapter 5 of Icons of Evolution.” The Talk.Origins Archive. Last modified 6 December 2006. Accessed on 14 July 2013 at http://www.talkorigins.org/faqs/wells/haeckel.html.
- Michael K. Richardson, James Hanken, Mayoni L. Gooneratne, Claude Pieau, Albert Raynaud, Lynne Selwood, and Glenda M. Wright. “There is no highly conserved embryonic stage in the vertebrates: implications for current theories of evolution and development.” Anat. Embryol. 196: 91-106 (1997). Accessed on 17 July 2013 at http://www.oeb.harvard.edu/faculty/hanken/documents/Richardson%20et%20al%201997%20Anat%20Embryol.pdf. [Archived here.]
- Michael K. Richardson and Gerhard Keuck, “A question of intent: when is a ‘schematic’ illustration a fraud?” Nature 410/6825: 144 (8 March 2001).
- “‘MAN-APES’ THE SUBJECT OF A WAR OF SCIENCE; Between Prof. Haeckel and Dr. Brass Rages a Controversy in Which Prehistoric Heads and Tails Are the Fruitful Themes.” The New York Times, 7 February 1909. Accessed on 16 July 2013 at http://query.nytimes.com/mem/archive-free/pdf?res=F30F11F83C5C15738DDDAE0894DA405B898CF1D3.
- J. Assmuth and Ernest R. Hull. Haeckel’s Frauds and Forgeries (Bombay: Examiner Press, 1915), pp. 14, 15.
- Douglas Theobald, Ph.D. “29+ Evidences for Macroevolution: The Scientific Case for Common Descent. Part 2: Ontogeny and Development of Organisms.” The Talk.Origins Archive. Last modified 17 May 2013. Accessed on 14 July 2013 at http://www.talkorigins.org/faqs/comdesc/.
- Daniel Gasman. “From Haeckel to Hitler: The Anatomy of a Controversy.” eSkeptic, 10 June 2009. Accessed on 14 July 2013 at http://www.skeptic.com/eskeptic/09-06-10/#feature. [Archived here.]