Authors of biology textbooks, when presenting what they regard as evidence for evolution, often include homologous organs, analogous organs, and vestigial organs in morphology as well as the theory of recapitulation in embryology. The organs of different organisms exhibiting likeness in structure due to evolutionary differentiation from the same or a corresponding part of a remote ancestor are called homologous organs.
Homologous organs are the same in their basic structure, though their shapes and functions may differ. For example, a human being's hands, a dog's front legs, and a whale's fins are homologous organs. The anatomical parts (organs) of different structure and origin showing correspondence in function are called analogous organs. (They came to have the same external shape and function as a result of their adaptation to the environment.) The wings (frontal legs) of the bird and the wings of the insect are examples of analogous organs. The organs of living beings that are considered to have functioned in their ancestral period but later to have lost their original functions in the evolutionary process are called vestigial (or rudimentary) organs.
When the embryos of vertebrates are compared with one another, all of them resemble one another in their early stages of development: All of them have gill slits and a tail, and all have a fishlike heart with a single atrium and ventricle. Based on that, evolutionists claim that embryos, in the course of development, repeat the evolutionary history of their ancestors in some abbreviated form. This is the theory of recapitulation, advocated by E. Haeckel (1834-1919), according to which "ontogeny recapitulates phylogeny."
Concerning this alleged evidence for evolution, many questions and refutations have been raised. Concerning homologous organs, Hitching raises the following question:
This latter [the tetrapod limb in vertebrates] is a classic textbook example of nature persuading one structure to do several jobs. Why should the legs of a horse, the wing of a bird, the arm of a man, and the flipper of a whale all be built the same way when serving quite different purposes? If the fittest adaptation were chosen by a gradual accumulation of mutations, you would have expected an organ used for flying and a organ used for running to have finished up-or even begun-looking totally dissimilar. (Hitching 1982, 149 -50)
Concerning analogous organs, the origin of their resemblance is still unknown. About this point, Komatsu says,
In the world of organisms, likeness often can be found in form and behavior among very remote species ... The inquiry into the cause of the likeness seen among remote species seems to be left far behind, partly because it is an extremely ambiguous matter, in a sense. (Komatsu 1982, 61-62)
Concerning homologous and analogous organs, B. C. Nelson said, from the Christian creationist position, that the likeness of these organs can serve as evidence for creation as much as it can serve as evidence for evolution.
Similarity in itself proves evolution no more than it proves creation. To the believer in the Bible the similarity of structure in living organisms merely establishes the fact that there was one Great Architect, or Creator, who, when He was about to build many of His species, had in mind one plan or pattern, and this He used for as many creatures as possible with such modifications of the general plan as were necessary for different conditions of existence. (Nelson 1967, 20)
Nelson also said that there are no such things as vestigial organs.
It is certainly not reasonable that the Creator would put into any one of His creatures parts that are absolutely of no use to it. Certainly He would not put in detrimental parts ... If a part serves any function whatever, whether it is only in the embryonic period, in the years of childhood growth, or later, that part is useful and cannot reasonably be considered a proof of evolution. (Nelson 1967, 42)
Nelson also mentioned that there are certain organs, the reason for whose existence was formerly unknown. Yet, with the progress of medical science, the functions of these organs have come to be known, and therefore, he said, they can no longer be called vestigial organs.
The theory of recapitulation has also been questioned. Often cited as evidence for the theory of recapitulation are the embryonic gill slits in human embryos, supposedly showing the fish stage of our ancestry (see Fig. 17). But according to Hitching, these are pharyngeal pouches rather than gills as such. In fish, they turn into gills; in mammals, into glands. "They seem, in fact, to be simply an essential and predictable stage of growth common to living embryos before they diverge on their genetically preordained pathways," Hitching says (Hitching 1982, 174).
In this way, there are various problems in what is claimed to be evidence for evolution in morphology and embryology. Nevertheless, those claims continue to be included in all textbooks of biology, as before.