230 THE EVOLUTION OF CONTINUITY 



But sources of Attraction and Repulsion would, in 

 demanding the formation of the Fish's fins, also demand 

 that these should be symmetrical in shape and situation, 

 and for the following reasons : Lines of attraction or repul- 

 sion are for all purposes straight lines, and unhindered 

 response along them in the form of movement towards the 

 source of attraction or away from that of repulsion must 

 be undeviating movement. Thus, the above forces, in 

 inducing through evolving nerve-centres the development 

 of motor appendages, demanded that these should in the 

 first place develop symmetry of power calculated to prevent 

 deviation. Such symmetry of power could be exerted by 

 two appendages, or sets of these, asymmetrical in form 

 and position ; but in the case of the developing primitive 

 Fish the body which was called on not to deviate was already 

 moulding on bilaterally symmetrical lines, so that the lateral 

 fins would naturally appear in symmetrical positions, and 

 each pair be symmetrical in form. The asymmetrical would 

 not be allowed to appear. 



In a word, local sources of attraction and repulsion 

 would cause the developing Fish to move, and to do this 

 to or from them in as straight a line as possible ; and it 

 would be they who made the Fish produce fins, and caused 

 these to appear in symmetrical pairs. During the responsive 

 movements of the developing Fish external physical forces 

 would constantly try to produce deviation from the line 

 of attraction or repulsion, and this would call for correction ; 

 and it would be the source of attraction or repulsion itself 

 which would correct such deviation as soon as it occurred, 

 and do so by means of a bilaterally symmetrical brain and 

 nervous system. 



We have indicated Food-attraction as the chief of the 

 local attractions which, in the face of, or at times with the 

 help of Gravity must have called for locomotion. But 

 sources of direct repulsion must also have been a powerful 

 factor. Repulsion is, however, but reversed attraction. If 

 the nerve-centre at the anterior end of a Fish is affected 

 by force of repulsion acting directly on it, it moves away 

 from the source as quickly as possible ; the organism swings 

 round, presents its tail to the source of disgust, and moves 



