436 MOKPHOLOGY. Chap. XIII. 



suppose that their common progenitor had an upper lip, 

 mandibles, and two pair of maxillae, these parts being 

 perhaps very simple in form ; and then natural selection 

 will account for the infinite diversity in structure and 

 function of the mouths of insects. Nevertheless, it is 

 conceivable that the general pattern of an organ might 

 become so much obscured as to be finally lost, by the 

 atrophy and ultimately by the complete abortion of cer- 

 tain parts, by the soldering together of other parts, and 

 by the doubling or multiplication of others, — variations 

 which we know to be within the limits of possibility. 

 In the paddles of the extinct gigantic sea-lizards, and 

 in the mouths of certain suctorial crustaceans, the 

 general pattern seems to have been thus to a certain 

 extent obscured. 



There is another and equally curious branch of the 

 present subject ; namely, the comparison not of the same 

 part in different members of a class, but of the different 

 parts or organs in the same individual. Most physio- 

 logists believe that the bones of the skull are homo- 

 logous with — that is correspond in number and in re- 

 lative connexion with — the elemental parts of a certain 

 number of vertebrae. The anterior and posterior limbs 

 in each member of the vertebrate and articulate 

 classes are plainly homologous. We see the same law in 

 comparing the wonderfully complex jaws and legs in 

 crustaceans. It is familiar to almost every one, that in a 

 flower the relative position of the sepals, petals, stamens, 

 and pistils, as well as their intimate structure, are intel- 

 ligible on the view that they consist of metamorphosed 

 leaves, arranged in a spire. In monstrous plants, we 

 often get direct evidence of the possibility of one organ 

 being transformed into another; and we can actually 

 see in embryonic crustaceans and in many other ani- 

 mals, and in flowers, that organs, which when mature 



