2i8 GRAND STRATEGY OF EVOLUTION 



in each metamere proceeds in a, b, c, d, order, to the 

 right and left of the median neural axis, the older 

 stages being nearest the median line, the youngest, far- 

 thest away, or nearest to the haemal surface; see dia- 

 gram (figs. 5 and 6). Thus with growth in volume, 

 there is a longitudinal, gradient increment from head to 

 tail ; transverse gradient increments to the right and left 

 of the neural axis towards the opposite ha3mal side; and 

 a radial gradient increment from the centre to the pe- 

 riphery. Hence the name, triaxial gradient increment. 



It is evident that no two points in such a growing 

 system can ever be alike, either in their historical, or 

 spacial relations; or in their functional conditions; or 

 in their potentialities. For every cross section will 

 display a condition a little younger than that in front 

 of it, and a little older than that behind it; and in every 

 cross section the lines drawn from a central point to 

 all the peripheral points will be divided into two mir- 

 ror-image sets, right and left; each set forming a graded 

 series of radial lines extending from the neural to the 

 haemal side, and each line representing a series of 

 points at graded distances from the centre of the body, 

 or from any object in the outside world, fig. 6. C. 



We may regard this triaxial system, if we choose, as 

 a system of points, graded in their potentialities, in ac- 

 cordance with the geometric properties of that system 

 and in accordance with their functional relations to a 

 permanent asymmetry in the outer world. In the liv- 

 ing body, these potentialities are expressed in archi- 

 tectural details arranged in right and left longitudinal 

 series, in right and left transverse series, and in over- 

 lying concentric strata. Hand in hand with these dis- 



