638 HISTORY OF SCIENCE. 



metatarsal and metacarpal bones) and of an equal number of toe and 

 finger-joints. This kind of resemblance has received the name of 

 serial homology. The homology of parts in a series may be strikingly 

 illustrated by instances in which the similarly constructed parts are 

 very numerous, as in the centipedes, two of which are represented in 

 Fig. 318. The common lobster furnishes a case in which the serial 

 homology may be traced under the modified forms. The six segments of 

 the so-called tail, show it clearly enough ; however, in the front part, the 

 segments in the adult animal coalesce ; but on looking at the under 

 surface of the body we find a series of pairs of movable appendages 

 taking on different forms. Such are the feelers, the jaws, the foot-jaws, 

 the claws, the legs, and the swimmerets attached to the segments of 

 the " tail." Now, although the segmentation is disguised in the fore 

 part of the lobster, there are species of closely allied forms in which 

 the segments remain more distinct. There is in general but little 

 (except the two pairof limbs) in the outward forms of vertebrate 

 animals to suggest a serial homology ; but the case is different when 

 we contemplate the structure of the skeleton, and especially that of 

 the back-bone. There the serial arrangement of vertebra having a 

 common conformation is conspicuous, and there are the similarly 

 constructed pairs of ribs. The skull appears to have no relation to 

 any serial arrangements, yet Goethe had at the beginning of the present 

 century arrived at the conception of the bones of the skull being 

 formed by modifications of the parts of a series of vertebrae. The 

 origination of this idea is, however, commonly attributed to LORENZ 

 OKEN (1779 1847), who describes how in 1806, when one day 

 rambling in the Hartz Forest, he came across the blanched skull of a 

 deer, and the thought flashed into his mind, "// is a vertebral column" 

 These ideas were afterwards taken up and developed, more particu- 

 lar!^; the eminent English comparative anatomist, Professor OWEN 

 (1804 . . . .), in his celebrated work entitled "On the Archetype 

 and Homologies of the Vertebrate Skeleton," published in 1848. 

 Oken's book on the " Philosophy of Nature " contains the first sug- 

 gestions of other conceptions which have in more recent times been 

 developed in biological science. For example, his Urschleim (" Pri- 

 mitive Slime ") is the protoplasm of recent observers, and (under 

 another name) he refers to the cell as the structural element of all 

 organisms. 



The doctrine of the descent of species by gradual transformation, 

 which has exercised so much of the thought of the present century, 

 may be traced in its rudimentary forms in the writings of several philo- 

 sophers and naturalists of last century. ERASMUS DARWIN (1731 

 1802) in his "Zoonomia," 1795 ; KANT (1724 1804), the great Ger- 

 man metaphysician, in a work published in- 1790 ; Goethe, in various 

 passages of his writings produced between 1780 and 1832; TRE- 

 VIRANUS, of Bremen (1776 1837), in his " Biology" (1802); Oken, 



