CHAP XIV.] DEVELOPMENT AND EMBRYOLOGY. 365 



supposes that the stripes on the whelp of a lion, or the spots on 

 the young blackbird, are of any use to these animals. 



The case, however, is different when an animal during any part 

 of its embryonic career is active, and has to provide for itself. The 

 period of activity may come on earlier or later in life ; but when- 

 ever it comes on, the adaptation of the larva to its conditions of 

 life is just as perfect and as beautiful as in the adult animal. In 

 how important a manner this has acted, has recently been well 

 shown by Sir J. Lubbock in his remarks on the close similarity of 

 the larvae of some insects belonging to very different orders, and 

 on the dissimilarity of the larvae of other insects within the same 

 order, according to their habits of life. Owing to such adaptations, 

 the similarity of the larvae of allied animals is sometimes greatly 

 obscured ; especially when there is a division of labour during the 

 different stages of development, as when the same larva has during 

 one stage to search for food, and during another stage has to search 

 for a place of attachment. Cases can even be given of the larvae 

 of allied species, or groups of species, differing more from each 

 other than do the adults. In most cases, however, the larvae, 

 though active, still obey, more or less closely, the law of common 

 embryonic resemblance. Cirripedes afford a good instance of this ; 

 even the illustrious Cuvier did not perceive that a barnacle was a 

 crustacean : but a glance at the larva shows this in an un- 

 mistakable manner. So again the two main divisions of cirripedes, 

 the pedunculated and sessile, though differing 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 must be considered as lower in the scale than 

 the larva, as with certain parasitic crustaceans. To refer once 

 again to cirripedes : the larvae in the first stage have three pairs 

 of locomotive organs, a 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 compound eyes, and extremely complex 

 antennas; but they have a closed and imperfect mouth, and cannot 

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

 developed organs of sense, and to reach by their active powers of 

 swimming, a proper place on which to become 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 



