DEVELOPMENT AND EMBRYOLOGY. 427 



we have parts commonly called homologous which bear no 

 relation to the descent of distinct species from a common 

 progenitor. Homoplastic structures are the same with those 

 which I have classed, though in a very imperfect manner, as 

 analogous modifications or resemblances. Their formation 

 may be attributed in part to distinct organisms, or to dis- 

 tinct parts of the same organism, having varied in an analo- 

 gous manner ; and in part to similar modifications, having 

 been preserved for the same general purpose or function, of 

 which many instances have been given. 



Naturalists frequently speak of the skull as formed of 

 metamorphosed vertebrae ; the jaws of crabs as metamor- 

 phosed legs ; the stamens and pistils in flowers as metamor- 

 phosed leaves ; but it would in most cases be more correct, 

 as Professor Huxley has remarked, to speak of both skull 

 and vertebrae, jaws and legs, etc., as having been metamor- 

 phosed, not one from the other, as they now exist, but from 

 some common and simpler element. Most naturalists, how- 

 ever, use such language only in a metaphorical sense ; they 

 are far from meaning that during a long course of descent, 

 primordial organs of any kind — vertebrae in the one case 

 and legs in the other — have actually been converted into 

 skulls or jaws. Yet so strong is the appearance of this 

 having occurred, that naturalists can hardly avoid employing 

 language having this plain signification. According to the 

 views here maintained, such language may be used literally ; 

 and the wonderful fact of the jaws, for instance, of a crab, 

 retaining numerous characters, which they probably would 

 have retained through inheritance, if they had really been 

 metamorphosed from true though extremely simple legs, is 

 in part explained. 



DEVELOPMENT AND EMBRYOLOGY. 



This is one of the most important subjects in the whole 

 round of natural history. The metamorphoses of insects, 

 with which every one is familiar, are generally effected 

 abruptly by a few stages ; but the transformations are in 

 reality numerous and gradual, though concealed. A certain 

 ephemerous insect (Chloeon) during its development moults, 

 as shown by Sir J. Lubbock, above twenty times, and each 

 time undergoes a certain amount of change ; and in this case 

 we see the act of metamorphosis performed in a primary 

 and gradual manner. Many insects, and especially certain 



