390 PROP. S. H. REYNOLDS AND DR. A. VATTGHAN ON [Aug. 1 9 I I , 



places the age of a bed beyond dispute — this is, indeed, the one 

 impregnable method of zoning. 



In the case of the Carboniferous corals, a splendid start has been 

 made by Mr. Carruthers, and, in this paper, I point out the main 

 stations along some other important lines of coral evolution. 



Among the graptolites Dr. Marr, followed by Miss Elles, has 

 pointed out several evolutionary lines which have a striking proba- 

 bility ; the suggested series, however, await the confirmation that 

 can only be assured by the determination of the ratios which two 

 successive mutations bear to each other numerically, from the date 

 of earliest occurrence of the second mutation up to the time when 

 it entirely replaces the first. 



(vi) The lines of evolution in the graptolites and in 

 the Palaeozoic corals. — As has been explained by Dr. Marr, the 

 graptolites progressed along two directions and in two stages : — 



(1) by the numerical reduotion of similar parts ; 



(2) by the increase of structural complexity in the remaining parts. 



This method of evolution is strictly comparable with that of 

 mammalian dentition. 



Furthermore, the greatest 'generic' variation took place during 

 early periods. Again this agrees with the case of the early Tertiary 

 mammals. 



In the Paheozoic corals, viewed broadly and as a whole, the first 

 stage finds a parallel in the reduction of fossulae resulting in the 

 development of radial from quadrantal symmetry. The second 

 stage is, however, the most important in the case of the corals, for 

 structural complexity, at the end of the Lower Carboniferous, rose 

 to a high pitch, resulting in a maze of so-called genera among the 

 Clisiophyllids. 



Thus, in the graptolites, the greatest ' generic ' variation took 

 place at the beginning of their history — in the corals, at the end. 



Reviewing all the foregoing facts, we see that extensive distri- 

 bution and constancy of zonal sequence are characters shared alike 

 by corals and by graptolites — both lowly organisms. Furthermore, 

 the corals with which we are concerned inhabited a narrow strip 

 along the shore-line : adult, the animal was sessile ; young, it was 

 at the mercy of currents, and it is at this stage that migration 

 was rapidly effected. 



The commonly accepted view that graptolites, either of themselves 

 or as passengers, lived in the open sea seems to need stronger 

 testimony than has yet been adduced, especially in view of the fact 

 that rapidity of variation is a phenomenon usually associated rather 

 with the fluctuating conditions of in-shore life, than with the 

 monotonous sameness of oceanic existence. 



