CHAPTER XXIV 

 SEGMENTAL BILATERAL SYMMETRY 



This is well illustrated by the typical Fish, and by terrestrial 

 vertebrates generally ; and as the latter have undoubtedly 

 derived their bilateral symmetry from some primitive 

 piscine type, it is to the modern typical Fish that we shall 

 turn our attention. 



In Chapter XIX it has been suggested that the segment 

 of the Fish has a medusoid derivation, and that the primitive 

 Fish was equivalent to a continuous series of medusoids 

 which, as the series tried to develop, were compressed to 

 become indivisible segments of one organism (see Figs. 57 

 to 62). As the plan of a medusoid is radially symmetrical, 

 it is not difficult to see that here we have the basis of piscine 

 symmetry. That is, we may regard the primitive Fish as 

 having had hereditarily transmitted to it the capacity of 

 developing as a cylindrical tube built up of segments, each 

 radially symmetrical. If this be admitted, the problem 

 which then remains is how such capacity or potentiality 

 came to be realised as the bilaterally symmetrical Fish, 

 The explanation which we have to offer is that Environment, 

 as modifying force, evolved the Fish's tapering form and 

 bilateral symmetry ; and that the two main ways in which 

 Environment acted were probably as Gravity, and sources 

 of attraction competing with this force. 



The force of Gravity, acting directly and in the form of 

 water-pressure, could never have evolved the piscine form 

 without the interference of other forces. Gravity, uninter- 

 fered with, would have gifted the results of its compression 

 with immobility, and while capable of moulding development 

 on some cylindrical plan, as in the case of the zooid and 

 anemone-megazooid, could never alone have produced 

 bilateral symmetry. But the primitive Fish must have 

 been actively motile from the first, and it is to this motility 



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