(691) 



XII. — Studies in Floral Zygomorphy. I. The Initiation of Staminal Zygomorphy. 

 By John M'^Lean Thompson, M.A., B.Sc, Assistant in Botany, University of 

 Glasgow. Communicated hy Professor F. 0. Bower, F.R.S. (With Two Plates.) 



(MS. received December 16, 1912. Read January 20, 1913. Issued separately September 20, 1913.) 



In the quest of phyletic lines, biologists who have based their opinions on the 

 widest field of observation and comparison known to their age have often felt that the 

 position which they have assigned to any organism within its own affinity may be 

 given to another by later systematists. 



The origin of this uncertainty of systematic position is not to be found in a fore- 

 boding that the immediate descendants of an organism, now considered relatively 

 simple, may suddenly acquire characters which will justify changes of position in the 

 atfinity. It is to be found in the biologist's avowed ignorance of the relative value of 

 the characters by which he judges, and in his belief that much which is important has 

 escaped his observation. 



It has followed that, according to the varied estimates of each character, so have 

 been the correlations on which systems have been founded. Further, the more the 

 observer foresees errors which may arise from the consideration of characters as isolated 

 things, the greater has been his felt need of developmental knowledge, for it has seemed 

 that by the developmental study of each part of an organism, and by the correlation 

 of these developments, a reasonable estimate of the condition of any part — during 

 development and as a functioning organ — may be formed. 



The limits within which this method has been followed are narrow, but a check has 

 been put upon any opinion, since, for each departure in form or function of any part 

 from that which the observer conceives to have been peculiar to the equivalent part 

 in the ancestry, modifications have been sought in the closely related parts. These 

 modifications are likewise recognised by comparison of those parts with their equivalents 

 in the pictured ancestor. 



Morphologically, these alterations may be slight. 



On the other hand, the observer may record no modification in form, but, when an 

 organism is considered developmentally as an assemblage of parts each of which has 

 reached a definite morphological stage at any point of time, a developmental study may 

 suggest some alterations in the intervals of time which separate successive develop- 

 mental stages in any part, and point to disturbances in the order of part-development. 



Here again the modifications outlined are recognised after comparison of the 

 developniental-time-intervals and order of part-development with those believed, by 

 the observer, to have been peculiar to the immediate ancestry. 



TRANS. ROY. SOC. EDIN., VOL. XLIX. PART III. (NO. 12). 93 



