70 SECTIONAL ADDRESSES 



short distance with curved transverse ribs upon which longitudinal ribs 

 are next superposed, thus giving to the ornamentation of the shell a can- 

 cellated appearance. This condition persists to the end of life. In 

 V. petrosus the curved rib and cancellated stages appear earlier in develop- 

 ment and are succeeded in late life by a spiny stage. This latter, in turn, 

 experiences an acceleration in the time of its appearance in later members 

 of the species and in V. sayana. Senile characters also exhibit a like 

 behaviour. 



At one time the ammonites were the citadel of recapitulationists. 

 Unfortunately so much scepticism is expressed by present-day specialists 

 on this group concerning the work of their predecessors that it is difficult 

 to find examples that are above suspicion. This is not, however, the 

 case with the detailed work done by Bisat (1924) on the goniatites. Though 

 he deliberately refrains from analysing his data from the standpoint of 

 the principles of development, in dealing with Reticuloceras reticulatum 

 he does allow himself to give, in tabular form, a valuable summary of his 

 observations upon the changes undergone by the ornamentation of the 

 shell in the course of the evolution of this species, and upon the times of 

 appearance of these changes during the development of successive 

 mutations. He thus provides us with yet another well-established case 

 of adult recapitulation. 



Among brachiopods many examples are forthcoming, but reference 

 must here be limited to Fenton's detailed work (1931) on evolution in 

 the genus Spirifer, a work that will repay careful study by all students of 

 palaeontology. The requirements of ample material in all stages of 

 development collected with meticulous care from minutely zoned strata 

 are sufficiently fulfilled to satisfy the most exacting critics. Space does 

 not permit of the description of specific examples here. We must there- 

 fore be content to quote Fenton's own words. Speaking of the two 

 gentes S. varians and S. obliqtiistriatus he says, ' there is also a close 

 correlation between ontogeny and phylogeny in each pair of trends. 

 Recapitulation is detailed, uniform, and generally involves the repetition 

 of adult characters. . . .' 



In the examples considered hitherto the study of the development has 

 been easy because, as growth proceeded, the early stages were not 

 destroyed but were retained, and the later stages were added to them. 

 With many other organisms this is not the case. Thus the trilobite as it 

 grew shed its skeleton, and with it all record of its juvenile features, at 

 more or less regular intervals, so that specimens of adult trilobites show 

 only adult features. Careful collecting from very fossiliferous beds may 

 result in the discovery of series in various stages of growth, and workers 

 who have collected such series claim, with good justification, to have 

 discovered in them evidence of the working of the principle of recapitula- 

 tion. But for our purpose, just at this moment such cases are not suffi- 

 ciently well authenticated. The same is true also for the echinoids which 

 reabsorb the earlier formed skeletal deposits as growth proceeds. 



On glancing back over the general survey of facts made above, one- 

 overruling condition seems to emerge, viz. that the more lowly and 

 simple the organism the more complete is the recapitulation. In more 



