SEXUAL DIMORPHISM 



It is known from embryological and palaeontological evidence 

 that free echinoderms are descended from animals which grow 

 like lilies at the end of fixed stalks, and such fixed animals 

 grow equally in the directions of a number of radii. We are 

 not justified in assuming that many other modes of growth 

 were possible and that radial symmetry has been selected 

 from a number of indefinite variations because it was the 

 most advantageous. On the other hand, we are justified in 

 concluding that the general plan of the echinoderm body has 

 been determined by the sessile condition of life, the growth 

 taking place equally in a number of directions from the 

 fixed centre, because the supply of nourishment and the 

 impacts of external forces are equal along those directions. 



The Ccelenterata, such as hydroids, sea-anemones, and coral 

 polyps, are also characterised by radial symmetry, and are 

 also fixed, and the same arguments apply to them. 



In the Mollusca we have essentially unsegmented animals 

 provided with an incomplete calcareous investment. In one 

 large tribe of Mollusca, the Gasteropoda, the shell and body 

 are twisted into a spiral. There is good reason to suspect 

 that the twisting of the shell during growth has mechanically 

 caused the torsion of the principal organs, intestine, nerve- 

 cord and heart, of the Gasteropod body. But no Darwinian 

 has succeeded in proving that the torsion is in any way 

 advantageous in the struggle for existence. 



The typical and primitive Vertebrate is the fish, from 

 which the terrestrial forms have been derived. The charac- 

 teristic vertebrate structures, axial skeletal rod, pharyngeal 

 gill-slits, fins, etc., are all adaptations in the defined sense. 

 But we need not be surprised to find that the diagnostic 

 structural features are here more obviously adaptational 

 than in other phyla, because the vertebrate gets its food by 

 active motion, and the whole body is an elaborate locomotive 

 mechanism. But it does not follow, because in one phylum 



