In the skeletal muscles of the primates many muscles offer themselves for consideration as examples of inherited structures arising under the stimuli of altered function, and only a few of these will be dealt with. It might appear sufficient to those who yield, perhaps too willingly, to authority, if I were here to try and prove my point by quoting the statements of one of the greatest anatomists of our time and country, and so pass on--but it is to be feared authority cannot carry one far in a dispute so important. Macalister says, however, “The anatomical arrangement of the muscular system is the physical exponent of habitual actions and those actions are the chief factors in moulding the bones and in regulating the position of the somato-pleural vessels and nerves”--and “the locomotory function and consequent utility of the trunk-muscles were lost when the early vertebrates became terrestrial. In higher vertebrates, and notably in man, the mobility of several regions of the vertebral column differs both in degree and kind: the outgrowing vertebrate processes show consequent variations, and _the muscular system is varied accordingly_.”[77] Also “as both origins and insertions (of muscles) are the creatures of habit, they are both equally variable with variation of function; but, as in higher animals the kind of work to be done is more constant than its degree, so, as a rule, insertions alter less than origins.”[78] Macalister, at any rate, held a very clear dynamical rather than static view of the making of the muscular system. But as the days of authority are in a certain sense gone for ever, and we live under the reign of experiment, research and questioning, every biologist, within certain limits, does what is right in his own eyes; there is no King in these days. [77] _Op. cit._, p. 71. (Italics not in original.) [78] _Op. cit._, p. 73. Skeletal muscles are structures in which, if ever, the factors of use and habit and disuse would be shown, because muscle is a tissue, with highly active metabolism, so that it has been called “an expensive tissue” for the animal to maintain. Muscles of Primates.