CONTENTS 13 



Chapter XXIII 



PAOE 



The Evolution of Symmetry 217 



Symmetry of Form accompanying Continuity is here dealt with — 

 The result of special factor, and one necessarily exercising sym- 

 metrical power — The symmetry of the Discontinuously Multi- 

 cellular Individual is that of Numbers ; this is not the basis of 

 Symmetry of Form in cellularly-continuous organisms. 

 Symmetry of Form finds its explanation in terms of Attraction 

 and Repulsion — The action of the former force more important — 

 Three ways in which Attraction has probably acted : as (1) Terres- 

 trial, (2) as Solar, and (3) as Food Attraction — The different ways 

 in which Terrestrial Attraction, or Gravity, has acted, and con- 

 tinues to act — Directly — Indirectly, as atmospheric and as water- 

 pressure — Specific Gravity — Solar and Food attraction frequently 

 compete with Gravity ; Gravity also competes with itself — The 

 share of Repulsion in producing Symmetry — Cells grow in Con- 

 tinuity not as the result of mutual attraction, but owing to external 

 compressing force ; owing to Gravity — The production of the 

 symmetry of the discontinuous Zooid; of the Megazooid. 



Chapteb XXIV 



Segmental Bilateral Symmetry 224 



Symmetry of Form in the Fish — Gravity, alone, could not have 

 made the Fish symmetrical ; Food-attraction has shared in the 

 process — Movement during development, in presence of water 

 pressure, would give the developing primitive Fish a symmetrically 

 tapering form, but could not unaided produce the bilateral sym- 

 metry the body actually exhibits — The influence of the digestive 

 tract and its contents in the matter — "Ventral ballast" — In all 

 vertebrates the abdominal viscera are pulled by Gravity to face 

 the earth's surface — The swimming-bladder and its functions ; its 

 absence in flat fish — The loss of equilibrium in the developing 

 flounder. 



Chapter XXV 

 Segmental Bilateral Symmetry (continued) . . . 229 



Terminal compression and the Fish's brain — Why this organ 

 develops at the anterior end of the organism — Sources of attraction 

 or repulsion which call for the response of Locomotion demand 

 that this be in as straight a line as possible to or from the source ; 

 they call for no deviation — The attracting and repelling forces 

 would demand the formation of locomotive appendages, and of 

 a kind capable of exercising bilateral symmetry of power — The 

 bilateral symmetry of site and shape of the fish's fins would thus 

 result. 



Repulsion induces movement as strongly as Attraction — Repulsion 

 from one point is equivalent to attraction towards another — In 

 movement the chief deviation to be corrected would be lateral — 

 How such correction would demand a duplicate and bilaterally 

 symmetrical brain — How the over-correction of deviation would 

 be avoided- — How a crossed motor nerve tract would be inevitable ; 

 the decussation of pyramidal tracts — The call for crossed sensory 

 tracts ; the decussation of optic tracts — Why there is a spreading 



