248 THE oRiuiy of species 



now exist, but from some common and simpler element. 

 Most naturjilists, however, use such language only m a 

 metaphorical sense; they are far from meaning that dur- 

 ing 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 ex{)lained. 



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 (Chloeou) during its devel- 

 opment, 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 metamor- 

 phosis performed in a primary and gradual manner. 

 Many insects, and especially certain crustaceans, show 

 us what wonderful changes of structure can be effected 

 during development. Such changes, however, reach their 

 acme in the so-called alternate generations of some of the 

 lower animals. It is, for instance, an astonishing fact 



