,396 ON THE ORIGIN OF SPECIES 



such special adaptations, the similarity of the larvaB or 

 active embryos of allied animals is sometimes much 

 obscured ; and cases could be given of the larvae of two 

 species, or of two groups of species, differing quite as 

 much, or even more, from each other than do their adult 

 parents. In most cases, however, the larvae, though 

 active, still obey, more or less closely, the law of com- 

 mon embryonic resemblance. Cirripedes afford a good 

 instance of this : even the illustrious Cuvier did not per- 

 ceive that a barnacle was, as it certainly is, a crustacean ; 

 but a glance at the larva shows this to be the case in an 

 unmistakable manner. So again the two main divi- 

 sions of cirripedes, the pedunculated and sessile, which 

 differ widely in external appearance, have larvae in all 

 their stages barely distinguishable. 



The embryo in the course of development generally 

 rises in organisation : I use this expression, though I 

 am aware that it is hardly possible to define clearly 

 what is meant by the organisation being higher or 

 lower. But no one probably will dispute that the 

 butterfly is higher than the caterpillar. In some cases, 

 however, the mature animal is generally considered as 

 lower in the scale than the larva, as with certain para- 

 sitic crustaceans. To refer once again to cirripedes : 

 the larvae in the first stage have three pairs of legs, a 

 very simple single eye, and a probosciformed mouth, 

 with which they feed largely, for they increase much in 

 size. In the second stage, answering to the chrysalis 

 stage of butterflies, they have six pairs of beautifully 

 constructed natatory legs, a pair of magnificent com- 

 pound eyes, and extremely complex antennae ; but they 

 have a closed and imperfect mouth, and cannot feed : 

 their function at this stage is, to search by their well- 

 developed organs of sense, and to reach by their active 

 powers of swimming, a proper place on which to be- 

 come attached and to undergo their final metamorphosis. 

 When this is completed they are fixed for life : their legs 

 are now converted into prehensile organs ; they again 

 obtain a well -constructed mouth ; but they have no 

 antennae, and their two eyes are now reconverted into a 



