496 3Ir. Joseph Jackson Lister [Feb. 15, 



centres and a body segment corresponding. These are, however, 

 transient features, which are entirely obhterated as development 

 proceeds. 



The only vicAv that appears to deal adequately with these 

 phenomena is that which recognises in the a]3pearance of these transient 

 fish-like and limuloid features in the development respectively of bird 

 and scorpion, the repetition of characters, which in some remotely 

 distant age of the world were those of their adult ancestors. 



We must, however, proceed with the caution. The varied groups 

 of the decapod Crustacea present in their development larval stages 

 differing widely from one another and from the adults. Yet, if we are 

 able in any case to form an estimate of phylogenetic relationships from 

 adult structure, it is certain that the decapod Crustacea are a natural 

 group descended from a common ancestor. We have to conclude 

 that the strikingly different features of the larvae are in many cases 

 not ancestral, but adaptive features titting them for diverse condi- 

 tions in the pelagic existence through which they pass before settling 

 down as littoral forms and acquiring the adult state. 



It appears then that a stage in early life different from that of the . 

 adult may be due to the repetition of ancestral features, but that it 

 may on the other hand be an adaptation fitting the young organism 

 to special conditions. 



To which category are we to refer the modes of growth of the 

 young shell of the multiform foraminifera ? 



With regard to the series of forms included in the genera Pener- 

 oplis, Orbiculina and Orlntolites Carpenter definitely held the view 

 that the modes of growth adopted in early life by the more complex 

 members of the series are phylogenetic repetitions of the arrangements 

 of the simpler forms of the series, from which they had developed — 

 that they are in fact repetitions in ontogeny of the phylogenetic 

 history. 



Carpenter's view would, the lecturer believed, more and more be 

 found to be correct. 



Another view of the significance of the multiform character of the 

 shells of Foraminifera has been propounded of recent years by Pro- 

 fessor Rhumbler, and a scheme of classification based on it has become 

 current, and found its way into text-books. The early stages of 

 the shells are here also regarded as of high phylogenetic significance, 

 but in a sense precisely tlie reverse of that of Carpenter. Professor 

 Pthund)ler regards the early ai'rangements of the chambers as repre- 

 senting not an ancestral type, but one towards which the race is ad- 

 vancing, while later in life the organism falls back on the old bad 

 ways of its forefathers. He thus regards the nuiltiform shells as 

 " ))hylogenetically degenerating " in the course of growth. The lecturer 

 expressed his dissent fi-om tliis view. 



.Two more examples of the multiform condition might be con- 

 sidered. 



