250 THE PHILOSOPHY OF BIOLOGY 



logical evidence cannot be obtained, we require proof 

 that the animal can be traced backwards in time, by 

 means of other characters, to some form in which the 

 missing structures reappear. The schemata are thus 

 the generalised or conceptual morphology of the phyla. 

 They are not the morphology of an individual organism, 

 but they include the morphology of the race. 



They are, Bergson says, themes on which innu- 

 merable variations have been constructed. Structural 

 elements may be suppressed, as when the notochord 

 disappears in the development of the individual 

 Tunicate, though it is present in the larva. Or elements 

 may disappear and become replaced by other structures, 

 as when the true molluscan gills are lost in the Nudi- 

 branchs and are replaced by the respiratory plumes. 

 They may be reduced to vestiges, as in the case of the 

 " pen " of the Squids, or the " cuttlebone " of the cuttle- 

 fish, remnants of the domed shell of the primitive 

 mollusc ; or in the appendix vermiformis of the human 

 being, a remnant of the voluminous caecum of the 

 herbivorous animal. Structures which were originally 

 distinct may coalesce, as when the greater number of 

 the primitively distinct segments of the thorax of the 

 crustacean fuse to form the " body " of the crab ; or 

 when the segmental ganglia of the same animal fuse 

 together to form the great thoracic nerve-centre. The 

 form and situation of a structure may vary within 

 wide limits : thus the digestive cavity of some Ccelenter- 

 ates may be a simple sac, as in the Hydra, but it may 

 be partially subdivided by numerous mesenteries as in 

 the zooid of the Corals ; or the simple tubular alimen- 

 tary canal in some Platyhelminth worms may be 

 bifurcated in others, triple-branched in others again, 

 or even provided with numerous lateral branches, as 

 in the more specialised species in the group. Organs 



