Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

It was very soon found from study of the anatomy of various species of organisms, and especially in studying the ontogenesis of their individual organs, that the structural plans of the bodies of even very dissimilar organisms within the individual taxa are extremely similar in their individual details. For example, amongst vertebrates, the human hand, the digging limb of a mole and the wing of a bird contain the same groups of bones (Fig. XXVII.4). We say that the individual bones in various species can be homologized, i.e. that they are homologous structures, homologues. In addition, these bones were formed in the course of ontogenesis by basically the same mechanisms from the same basic forms. Simultaneously, if we compare the limbs of other groups of organisms, for example the limb of a mole and the limb of a mole cricket, it is immediately obvious that functionally identical types of limbs can be formed by completely different means, from completely different parts and through completely different ontogenetic mechanisms.
The occurrence of homologous structures within individual monophyletic taxa is now considered to be a consequence of the fact that the individual species were formed by divergence from a common ancestor. As biogeography has demonstrated that daughter species can be formed only from some parent species, comparative anatomy has also confirmed that a new anatomical structure can be formed only by modification of a different structure that already existed in the parent species.
The existence of rudiments and atavisms also confirm that the occurrence of homologous structures actually does not have a functional cause. Rudiments are residual organs that occur in all the individuals of a certain species, where they do not fulfill any function in this species. Thus, they are mostly much smaller and structurally simpler than where they fulfill some sort of function (Fig. XXVII.5). Some rudiments in the given species of organism are formed only during embryonic development and cannot be found in adult individuals. The appendix, a worm-like protuberance of the intestine, is considered to be a rudiment in humans. In a great many taxa, the appendix plays a very important role in digestion and is quite large (Fig. XXVII.6). It has been substantially reduced in humans and its removal apparently does not negatively affect the fitness of the individual.
Atavisms represent a similar case to rudiments. Atavisms are structures that, in contrast to rudiments, occur in only some individuals of a particular species, have no functional importance for their bearers and simultaneously occur in all the members of some other species, where they do have functional importance. For example, in humans, individuals occur very rarely that have an atavism in the form of a tail (Fig. XXVII.7). The occurrence of atavisms is once again very good evidence for the evolutionary mechanisms of the origin of species. The organisms can manage quite well without these structures, so that their presence has no functional basis. They are not formed in most individuals, so that their presence necessarily cannot follow from the properties of the components from which the body of the organism is formed or from the character of the processes of ontogenesis (see the structuralist explanation of evolutionary trends, XXVI.7.4). The only reasonable explanation that remains is the evolutionary explanation – the occurrence of rudiments and atavisms follows from the fact that these organs existed in functional form in an ancestor of the given species, so that the ability to form them is also borne by the members of successor species.