238 GRAND STRATEGY OF EVOLUTION 



are possible, for this long series of organic adjustments 

 has reached its logical conclusion. 



It is evident, for example, that the chief segmen- 

 tal sense organs, such as the visual, auditory, and ol- 

 factory organs, cannot be reduced to less than a single 

 pair, for a single pair, bilaterally symmetrical, as we 

 shall presently show, is absolutely essential to the per- 

 formance of their orienting functions. Nor can they 

 be more centralized around a common point of intake, 

 or in a position more suitable for head-first locomotion. 



On the other hand, organs like the locomotor ap- 

 pendages, which in primitive invertebrates may be 

 situated at the anterior end of the body, or arranged like 

 many pairs of oar-like legs on either side, or as two 

 pairs nearer the centre of the body, as in fishes, rep- 

 tiles, birds, and mammals, cannot be advantageously 

 reduced to less than a single pair, nor moved farther 

 back than they are in man. The same is true of the 

 excretory and reproductive organs. And finally the 

 necessarily voluminous lungs, heart, liver, and diges- 

 tive organs cannot be more centralized, or have their 

 dead weight better placed, or balanced, for safety and 

 economy of locomotion than they are at present. 



I have tried to show the chief events in this rise and 

 decline of metamerism in a series of diagrams, figs. 7 

 and 8. 



V. Architectural Tropism or the Orientation of Tri- 

 axial Organic Gradients to the Triaxial System 



of Gradient Action in the Outer World 



i 



We have indicated in preceding sections some of the 

 exceptional advantages which give triaxial growth its 

 impressive aspect of constructive Tightness. Or, other- 



