394 DEVELOPMENT Chap. XIII. 



itwouldinmosl cases probably be more corrcrf, as Prof. Huxley 

 has remarked, to speak of both skull and vertebnc, both jaws 

 and lef^s, etc., as having been metamoqihosed, not one from 

 the other, as they now exist, but from some common and 

 simpler clement. Most naturalists, however, use such lan- 

 guage only in a metaphorical sense ; they are far from mean- 

 ing that, ckiring a long course of descent, primordial organs 

 of any kind — vertebra? 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 nat- 

 uralists 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 charac- 

 ters, Avhich they probably would have retained through inheri- 

 tance, if they had really been metamorjihosed from ti-uc though 

 extremely simple legs, is explained. 



Develojinient and Emhryology. 



This is one of the most important subjects in the whole 

 round of natural liistor}^ The metamorphoses of insects, Avitli 

 which every one is familiar, are generally eflectcd 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. 

 Lubljock, about twenty times, and each time undergoes a cer- 

 tain amount of change ; and in this case avc see the act of 

 metamorphosis performed in a primary and gradual manner. 

 Many insects, and especially certain crustaceans, show us what 

 woiulerful changes of structure can be etTected during develop- 

 ment. Such changes, however, reach tlieir climax in the so- 

 called alternate generations of some of the lower aTiimals. It 

 is, for instance, an astonishing fact that a delicate branching 

 coralline, studded with polypi and attached to a submarine 

 I'ock, should jiroduce, first by budding and then by transverse 

 division, a host of huge floating jelly-lishes ; and that these 

 should ])roduce eggs, from which are hatched swimming ani- 

 malcules, which attach themselves to rocks and become devel- 

 oped into branching corallines ; and so on in an endless cycle. 

 The belief in the essential identity of the process of alternate 

 generation and of ordinary metamorphosis has been greatly 

 strengthened by Wagner's discovery of the larva or maggot 



