Practical anatomy of the rabbit : an elementary laboratory text book in mammalian anatomy / by B.A. Besley.

  • Bensley, B. A. (Benjamin Arthur), 1875-1934.
Date:
1921
Catalogue details

Licence: In copyright

Credit: Practical anatomy of the rabbit : an elementary laboratory text book in mammalian anatomy / by B.A. Besley. Source: Wellcome Collection.

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    With succession has also come modification, the evidence of which lies not only, geologically speaking, in the relative times of appearance of life forms on the earth, but also in the fact that succession leads from primitive to specialized animals, revealing in a large way the same kinds of differences observable among those living at the present day. That the entire skeleton of a mammal is patterned upon the primitive skeleton of the fossil amphibia of the Carboniferous and Permian is evident from a comparison of the components part for part, but it is equally evident from com- parative anatomy that the viviparous condition of a higher mammal is founded upon an oviparous condition in lower forms even if no fossil evidence is forthcoming. That a mammal as an air-breathing vertebrate should develop gill structures in the embryonic con- dition, though circumstances never come about by which such structures are used, is in itself an important fact bearing on adult structure, but such a condition also shows to what extent a living animal carries ancestral features, whether functionally modified or not. All characters of animals have thus an evolutionary basis, the general nature of which is easily understood although the process by which they have been developed is still a matter of uncertainty. In comparison with one another, animals present certain resem- blances and differences—-diagnostic features, which are used as a basis for classifying them into major and minor groups. In many cases characters of resemblance have been shown to be secondary, and are hence described as convergent. In some of these the resemblances are of a gross type, and the structures are described as analogous; in other cases they are exact or homoplastic. As a rule, however, characters of resemblance are broad marks of affinity, comparable to those seen on a small scale in human families, or in human races, and determined as in the latter cases by heredity. The chief basis of comparison of animals with one another is the general assumption that structures which are similar or identical are homogenous—of common origin On the other hand, their differences are chiefly marks of divergence in evolution. Although it is conceivable that many of the internal features of animals are the result of a general progressive development, more