201 



where several isomerides may occur simultaneously, is also observed 

 on many occasions by chemists, and it is a wellknown fact for in- 

 stance, how easily the threefold symmetrically substituted deri- 

 vatives of phenoles, aniline, etc. are commonly produced, in compari- 

 son with their less symmetrical isomerides (cf. page 202). 



On the contrary, evolution in living nature seems to proceed in 

 exactly the opposite direction, the lower animals showing in many 

 cases a much higher symmetry than the mere bilateral one of the 

 animals, which have appeared in the later periods of the earth's 

 history. Even a certain preference for pen- 

 tagonal symmetry, both in the case of 

 animals and of plants, seems to exist 

 here, - - a symmetry so closely related 

 to the important ratio of the "golden 

 section," and impossible in the world of 

 inanimate matter. 



The view, that really the older forms 

 should possess the higher symmetries, is 

 probably also sustained by the remark- 

 able phenomena of the occurrence of 

 so-called peloria l ) in flowers. 



It has been observed for a long time 

 that many plants, the flowers of which 

 have only bilateral symmetry, suddenly 

 produce at the top of an inflorescence a 

 flower which shows the perfect symmetry 



of one of the axial groups, or of the groups C^. Thus Delphinium 

 peregrinum produces occasionally a completely pentagonal blossom ; 

 the common foxglove (Digitalis purpurea monstrosa) exhibits the 

 same phenomenon (fig. 158), as the accompanying figure (after 

 H. de Vries) clearly shows. Among Orchidaceae the species Cattleya 

 marginata and Phalaenopsis Schilleriana occasionally show a pelo- 

 rium of perfect ternary symmetry. 2 ) This remarkable phenomenon 

 is commonly observed in the flower which stands at the apex of a 



Fig. 158. 



Pelorium of Digitalis 

 purpurea monstrosa. 



J ) From: niiup = monstrum. 



2 ) M. T. Martens, Nat. Hist. Review, (1863); Vegetable Teratology (1869). 



On symmetrical arrangement in the case of artificial situs inversus produced 

 by constriction of the eggs of Triton cristatus, and on its explanation, cf. : 

 H. Spemann and H. Fal ken berg, Archiv fur Entwickelungsmechanik 

 der Organismen, 45, 371, (1919). 



