Series - "Icons of Evolution, Haeckel's Embryos, Pt 2"

Last week I posted the topic titled "Haeckel's Embryos". We received a comment stating that I never explained what Ernst Haeckel's theory was. During this post we will be looking at what it was.

Ontogeny Recapitulates Phylogeny
Ontogeny: "the development of an individual from a fertilized ovum to maturity, as contrasted with the development of a group or species ( phylogeny )"

Recapitulate: (verb)

1. to recap (formal)
2. to repeat stages from the evolution of the species during the embryonic period of an animal's life

Phylogeny: (noun) the development over time of a species, genus, or group, as contrasted with the development of an individual ( ontogeny )

Ontogeny Recapitulates Phylogeny, also known as "Biogenetic Law" is not a "Law" of Science at all. It is only a theory.

A theory that was proven wrong many years ago.

At the top of this chart you will see how Haeckel drew each embryo. But at the bottom you will see a real picture of what each embryo really looks like.
Haeckel's Biogenetic Law 
Haeckel's Biogenetic Law maintains that vertebrate embryos pass through stages in which they exhibit adult features of their evolutionary ancestors. In its most famous example, the law teaches that "gill slits"
in vertebrate embryos reveal their common aquatic ancestry. But human embryos do not really have
gills or gill slits: like all vertebrate embryos at one stage in their development, they possess a series of
"pharyngeal pouches," or tiny ridges in the neck region. In fish embryos these actually go on to form
gills, but in other vertebrates they develop into unrelated structures such as the inner ear and parathyroid gland. The embryos of reptiles, birds and mammals never possess gills. 


Their evolutionary ancestorsis false and was already discredited in Darwin's lifetime. Nineteenth-century embryologist Karl Ernst von Baer pointed out that although vertebrate embryos resemble each other at one point in their development, they never resemble the adult of any species, present or past (von Baer 1828; Bowler 1989, p. 129). Prominent 20th-century embryologists have also criticized the Biogenetic Law: In 1922 Walter Garstang wrote that "the basis of this law is demonstrably unsound," and in 1958 Sir Gavin de Beer called it "a mental strait-jacket which has had lamentable effects on biological progress" (Garstang 1922, p. 81; de Beer 1958, p. 172).
 Although vertebrate embryos never resemble the adults of any species, it is true that they pass through an intermediate stage in which some of them superficially resemble each other (Haeckel's first stage). Looking at development from this intermediate stage onward, von Baer concluded that early embryos exhibit features common to the phylum before developing the distinguishing characteristics of classes, genera and species (von Baer 1828). 
Many 20th-century biologists prefer von Baer's interpretation to Haeckel's: Early embryos may not possess ancestral adult structures, but their similarities are interpreted as vestiges of ancestral embryonic features. Since Haeckel's drawings can be used to illustrate von Baer's interpretation as well as Haeckel's, they have survived even though the latter has been discredited. Haeckel's embryos have thus become familiar to generations of biology students. Unfortunately, his drawings misrepresent the facts.


Strive to have an Intelligent Faith!!!

-Nelis