Comparative Anatomy: The similarity between different species was one of the fundamental reasons for the development of the theory of evolution. In a sense comparative embryology and comparative anatomy are one and the same study, differing only with respect to the stages at which the organisms are studied. There are apparently two major reasons for similarities between species – heritage and habit. Heritage refers to a common ancestry, with similar genetic systems responsible for the resemblances. However, species with similar modes of life, with similar habits are often very much alike even though not closely related. Structures that are similar because of similar function or habits are said to be analogous and homology rests on a similar developmental origin and hereditary basis.
Characteristically there are seven cervical vertebrae in the mammalian neck. A mouse, an elephant and even a giraffe has the same number of cervical vertebrae. To the obvious question as to why animals differing so greatly in size, in structure, and in mode of life should have the same number of vertebrae in their necks, the theory of evolution presents a simple, plausible answer. All these varied forms, and the many other mammals, are descended, with modifications, from an ancestral mammalian stock that was characterized by seven cervical vertebrae.
Morphological homologies are actually based on homologies in the hereditary materials, genetic homologies, of which they are the most obvious manifestations. The genetic homologies have been based not only in the similarities of phenotypes but on the locations of these genes in the homologous regions of the chromosomes. The existence of many organs diverse in function and yet clearly similar in structure – for example, the human hand, the seal’s flipper, and a bat’s wing – is a difficulty best explained by evolution. The list of morphological homologies can be almost endlessly extended, but the interpretation remains the same: descent with modification.
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