392 THE ORIGIN OF SPECIES 



parts. If we suppose that an early progenitor — the archetype, as 

 it may be called — of all mammals, birds, and reptiles, had its 

 limbs constructed on the existing general pattern, for whatever 

 purpose they served, we can at once perceive the plain significa- 

 tion of the homologous construction of the limbs throughout the 

 class. So with the mouths of insects, we have only to suppose that 

 their common progenitor had an upper lip, mandibles, and two 

 pairs of maxillae, these parts being perhaps very simple in form; 

 and then natural selection will account for the definite diversity 

 in the structure and functions of the mouths of insects. Neverthe- 

 less, it is conceivable that the general pattern of an organ might 

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

 and ultimately by the complete abortion of certain parts, by the 

 fusion of other parts, and by the doubling or multiplication of 

 others, variations which we know to be within the limits of possi- 

 bility. In the paddles of the gigantic extinct sea-lizards, and in the 

 mouths of certain suctorial crustaceans, the general pattern seems 

 thus to have become partially obscured. 



There is another and equally curious branch of our subject; 

 namely, serial homologies, or the comparison of the different parts 

 or organs in the same individual, and not of the same parts or 

 organs in different members of the same class. Most physiologists 

 believe that the bones of the skull are homologous — that is, cor- 

 respond in number and relative connection — with the elemental 

 parts of a certain number of vertebrae. The anterior and posterior 

 limbs in all the higher vertebrate classes are plainly homologous. 

 So it is with the wonderfully complex jaws and legs of crustaceans. 

 It is familiar to almost every one, that in a flower the relative posi- 

 tion of the sepals, petals, stamens, and pistils, as well as their inti- 

 mate structure, are intelligible 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, during the 

 early or embryonic stages of development in flowers, as well as in 

 crustaceans and many other animals, that organs which when 

 mature become extremely different are at first exactly alike. 



How inexplicable are the cases of serial homologies on the ordi- 

 nary view of creation ! Why should the brain be enclosed in a box 

 composed of such numerous and such extraordinarily shaped 

 pieces of bone, apparently representing vertebrae? As Owen has 

 remarked, the benefit derived from the yielding of the separate 

 pieces in the act of parturition by mammals, will by no means 

 explain the same construction in the skulls of birds and reptiles. 



