55 







that liriiiilicdi.il development nii^ht only be a result of 

 lieular circumstances durin \vth >i the crystal, a view 



idi can, li"\vc\vr. hardly be thought justified. As we >\\;i\\ Bee 

 >tals have in many cases also a tendency to syncrystallise 

 apparently hijJiiT symmetrical aggregations than they arethun- 

 and thus often to imitate a symmetry of the composed 

 li \iclual obtained by polysynthetic twinning, much higher than 

 own. 



In living nature on the other hand, it seems that the morphological 

 ,-olution goes in the direction from higher to lower symmetry. 

 Whether it be so or otherwise: in any case the fact of the 

 inu-ral occurrence of the bilateral symmetry among higher animals 

 not yet been explained in a rational way. Although therefore 

 deeper causes of this phenomenon are not yet known, suspicion 

 that they are in some way connected with the particular 

 rcumstance of animal motion. Among the lower beings there are 

 ly, of which a sedentary life is characteristic; and it may be of 

 Ivantage for the purpose of nutrition, etc., for the living being, 

 be in contact with its environment in as many directions 

 is possible. Evidently the symmetrical repetition of parts or 

 rgans of its body, is a way of accomplishing this. In general the 

 impbility of the living organism is a factor much more compa- 

 )le with the real character of symmetry than motion is. For that 

 >n the organs of plants are arranged as a whole with higher 

 imetry, and are also more symmetrical in themselves than those 

 the animals, while the most perfect symmetry finally is mani- 

 sted in the forms of immobile, crystallised matter. On the other 

 md, for the animals which can move freely, the best mechanical 

 ibility may have been a reason of importance in the development 

 their somatic forms. l ) 



Finally, if n = 2 t the symmetry of the group C n consists in the 

 resence of the inversion as the only symmetry-property. 

 Such figures have thus only a centre of symmetry. As an instance 

 of this kind every crystal-form of a holohedral triclinic crystal, e.g. 

 of copper-sulphate ( + jH 2 0) in fig. 56, can be mentioned. It is more 

 difficult to find examples of living beings showing the symmetry 

 of this group C 2 , which is commonly indicated by the symbol /. 



*) Cf. F. M. Jaeger, Over Kristallografische en Molekulaire Symmetric van 

 plaatsings-isomere Benzol-derivaten, Dissertatie Leiden, (1903), p. 202 208; 

 Zeits. f. Kryst. 38. 592. (1904). 



