386 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 



adduces the close resemblance of the parts on the right and left 

 sides of the body, and in the successive segments of the same indi- 

 vidual animal ; and here we have parts commonly called homo- 

 logous, 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 forma- 

 tion may be attributed in part to distinct organisms, or to distinct 

 parts of the same organism, having varied in an analogous manner; 

 and in part to similar modifications, having been preserved for 

 the same general purpose or function, — of which many instances 

 have heen given. 



Naturalists frequently speak of the skull as formed of metamor- 

 phosed vertebra); the jaws of crabs as metamorphosed legs; the 

 stamens and pistils in flowers as metamorphosed leaves; but it 

 would in most cases be more correct, as Professor Huxley lias 

 remarked, to speak of both skull and vertebra?, jaws and legs, &c, 

 as having been metamorphosed, not one from the other, as they 

 now exist, but from some common and simpler element. Most 

 naturalists, however, 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 — vertebras 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 arc 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 per- 

 formed in a primary and gradual manner. Many insects, and 



