XVI 



THE PROBLEM OF PHYLOGENY 



1021 



earlier edifice; Anglo-Saxon land-tenure influences the planning of 

 our streets, and the cliff-dwelling and the cave-dwelling linger on 

 in the construction of our homes! So we see enduring traces of 

 the past in the living organism — landmarks which have lasted on 

 through altered functions and altered needs ; and yet at every stage 

 new needs are met and new functions effectively performed. 



When we consider (for instance) the several bones in a fish's 

 shoulder-girdle — clavicle, supra-clavicle, post-clavicle, post-temporal 

 and so on — and recognise these in this fish or that under countless 

 minor transformations, we have something which is not only wide- 



Fig. 490. Skeleton of moonfish, Vomer sp. From L. Agassiz. 



spread but is rooted in antiquity, and whose full significance seems 

 beyond our reach. But take the skeleton of some particular fish, a 

 moonfish or a John Dory will do very well, and look at its shoulder- 

 girdle from the mechanical point of view. It is a deal more than is 

 needed for the support of the small, weak pectoral fin; but another 

 function, and its perfect adaptation for that function, are not hard to 

 see. The flattened body of the fish is built (as we have seen also in the 

 plaice) on the plan of a parabolic girder; but out of this girder a 

 great gap has had to be cut, to hold the viscera. The great shoulder- 

 girdle serves to strengthen and complete the girder, to bind its 

 upper and lower members together, and to compensate for the part 



