55 



Fig. 56. 

 Potassium- 

 tetrathionate. 



In morphology this important symmetry is commonly indicated 

 as that of bilateral-symmetrical forms. As such, innumerable leaves 



(vine, oak, etc.), and flowers 



(Papillionaceae, Orchidaceae, 



Labiates, Fumaria, etc.) are 



representatives of this class. 



In fig. 58 a few flowers of 



this kind : Paphiopedilum 



Hookerae(d)', Galeopsis dubia 



(a); Lamium purpureum(b)\ 



Brunella grandiflora(c)', Dal- 



housiea bracteata(e) ; Cyclopia 



genistoides(f) ', Podalyria cor- 



data (g), have been reprodu- 

 ced as typical examples of the said symmetry ; 



they may bring back this wellknown fact 



to memory. 



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

 presence of the inversion as the only symmetry-property. 



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

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

 of copper-sulphate (+ 5H 2 0) in fig. 57, maybe 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 /. 



Perhaps some representatives of the 

 family of the Radiolaries, as e. g., Aulo- 

 sphaera elegantissima, may be conjectured 

 to possess this symmetry. 



6. It may be remarked here as a very 

 important fact, that almost all the higher 

 living beings (man, quadrupeds, birds, 

 insects, etc.) have the symmetry of the 

 group S, a single plane of reflection being 

 their only symmetry-element. The lower animals on the contrary 

 (Blastoidea, Coralla, Radiolariae, Discomedusae, Ascidiae, etc.) often 

 show a much higher symmetry characterised by axes of high period- 

 numbers (n = 4, 5, 6, etc.), and by the presence of several planes 

 of symmetry. In this respect it would appear, as if a striking 



Fig. 57. 

 Copper- sulphate (5f/ 2 0) . 



