THE EVOLUTION OF SYMMETRY 221 



directly opposing and distinct from Gravity was a factor, 

 namely, Solar Attraction. Moreover, this force would be 

 a factor calculated to work for immobility. As regards 

 submarine life, it is true that its power would vary inversely 

 with the depth of the water, but in shallow depths it would 

 be considerable. Its full power would come into play in 

 aerial environment, and its results are visible in the 

 immobility and habit of existing plants and trees. For 

 solar attraction at its strongest is at right angles to the 

 earth's surface, and a developing organism peculiarly sensi- 

 tive to its action would during its growth be subjected to 

 attraction in opposite directions. Thus, in the case of the 

 plant, Gravity is pulling along a line towards the earth's 

 centre, while the sun is pulling in an exactly opposite 

 direction. Other things apart, this would make for serial 

 extension, and also for immobility. Assisting Gravity in 

 producing and maintaining immobility in the case of the 

 plant, there would, of course, be the attraction exercised 

 by water and nourishment in the earth on the root. 



Solar attraction, however, is not the only force competing 

 with Gravity, and is probably not the fundamental one 

 upon whose action the motility or immobility of evolved 

 organisms has depended ; for this we believe to have been 

 Food-attraction, and minor forms of local attraction. 



When an organism has a fixed habit, its food invariably 

 surrounds it, or is brought within its reach by causes external 

 to the organism. In such cases, Food-attraction, as a force 

 demanding the response of motility, does not exist for the 

 organism, and Gravity and solar attraction successfully hold 

 the organism in situ. But in many cases the attracting 

 food is not brought within reach of the organism, and may 

 itself be immobile ; it may be patchy in its distribution, 

 and the patches may be situated at considerable distances 

 from each other. The organism then, as the attracted 

 entity, has to respond to the source of attraction by calling 

 into play its locomotive powers, which themselves originally 

 evolved through this habitual call for responsive movement. 

 And responsive movement of this sort has invariably to 

 be manifested in the " presence " of the force of Gravity. 

 Clearly, Gravity might assist movement, but in many ways 

 it would always be trying to prevent it. On the whole, 



