A. R. Horwood — Upper Trias of Leicestershire. 205 



parts as their progenitor, but these parts will be in diverse stages of 

 evolution, some having progressed rapidly, others slowly, some having 

 remained stationary, and others having been retrogressive. 



Acceleration and retardation become, then, as important factors in 

 phylogeny as they are recognized to be in ontogeny, and, as in the 

 latter case, concern the separate parts of the individual rather than 

 the complete organism. 



To sum up, the true phylogenetic position of a genus (or any other 

 rank) can be expressed only after its features have been separately 

 analysed and their various stages of progress appreciated. The 

 conception can be made clearer by thus expressing (for a few 

 characters) the relative positions of Lanieria, Discholectypus, and 

 Conuhis. 



E = retarded evolution. 



Ea = retarded, but sliglit progress. 



A 

 rA 



accelerated evolution, 

 progress, but not complete. 



In tracing the condition of phylogenetic development of a group 

 it is necessary, then, to have regard to three points : (1) an analysis 

 of the individual and a ' diagnosis ' of each separate structure ; 



(2) a knowledge of the corresponding features of the original member 

 of the group (sometimes available, often partly hypothetical) ; and 



(3) a knowledge of the ultimate possibilities of each structure in the 

 coui'se of its evolution (hypothetical, but to be roughly ascertained 

 by considering the most highly developed conditions found in the 

 group). Care must, of course, be taken to eliminate, as far as 

 possible, conditions which are the direct results of adaptation to 

 environment. 



IV. — The Uppek Tkias of Leicesteeshire. 

 By A. E. HOEWOOD. 



{Continued from the March Number, p. 121.) 



(5) The Rh(Btic Outcroi). 



ALTHOUGH the Ehsetic beds are not exposed continuously along 

 the eastern boundary of the Keuper outcrop, they have been 

 proved at many points from the Eiver Trent in the north on the 



