MOVEMENT 



the lowest skull-less vertebrates, the acrania, the in- 

 ternal skeleton of which consists merely of a cylindrical, 

 solid, and elastic axial rod {chorda), we see on each side 

 a row of muscular plates (fifty to eighty in the amphi- 

 oxus). In this case there are not pairs of limbs, and it is 

 the same with the oldest craniate animals, the cvclostoma 

 (myxinoida and petromyzonta). It is only with the 

 third class of the vertebrates, the true fishes {pisccs), 

 that two pairs of lateral limbs appear — the breast-fins 

 and belly-fins. From these, in their terrestrial descend- 

 ants, the oldest amphibia of the Carboniferous Period, 

 the two pairs of jointed legs — fore-legs (carpomela) and 

 hind-legs (tarsomela) — are derived. These four lateral 

 five-toed legs have a very characteristic and compli- 

 cated articulation, both in the internal bony skeleton 

 and the muscular system that encloses this and is at- 

 tached to it. From the amphibia, the earliest quadru- 

 peds, this locomotive apparatus is transmitted by hered- 

 ity to their descendants, the three higher classes of the 

 vertebrates, reptiles, birds, and mammals. As I have 

 dealt with these important structures fully in my .4;/- 

 thropogeny (chapter xxvi.), and given a number of illus- 

 trations of them, I must refer the reader to that work,' 

 and will only make a few observations on the mam- 

 mals. 



Both parts of the motor apparatus, the internal bony 

 skeleton (the passive supporting apparatus) and the 

 external muscular system (the active motor), exhibit a 

 great variety of construction within the mammal class, 

 in consequence of adaptation to the most different habits 

 and functions. We have only to compare the running 



* A translation of the latest edition of \y\e Anthropogenic, with 

 the full number of frrsh illustrations (thirty plates and five hun- 

 dred and twelve wood-cuts), will be issued very shortly by the 

 Rationalist Press Association, under the title of The Evolution 

 of Man. 



283 



