D.— ZOOLOGY. Hli 



which nothing else depends : it can be pruned ofi without injury to the 

 rest of the series. 



But we have also seen that the cell-lineages leading to certain adult 

 organs may differentiate so quickly as to make the rudiments of these 

 organs manifest in the trochosphere of which originally they did not form 

 a part. What will happen if these partly differentiated rudiments should 

 be capable of useful modification subservient to larval as distinct from 

 adult ends ? They will, ex hypothesi, be subject to the unity of the 

 inheritance, and if the modification be irreversible, i.e. incapable of 

 subsequent rectification, the adult form of the same organ will inevitably 

 be affected. Thus some modifications of adult characters may be the 

 result of larval mutations. Is there any evidence that such is ever the 

 case ? I believe such evidences are widespread, and that it is only the 

 dominance of an erroneous hypothesis which has prevented us from 

 recognising them before. Let me submit one or two examples in Mollusca 

 for your consideration. 



The systematic study of Mollusca has resulted, like that of other 

 groups, in the production of a classification based on the principles of 

 ' adult seriation.' Groups and sub-groups are defined ostensibly by their 

 possession of certain combinations of positive characters ; but the real 

 basis is the occurrence of gaps, some large and deep, others slight, in the 

 series of adults available for examination. As knowledge increases, these 

 gaps are often reduced or filled up, and the positive characters defining 

 the groups are then altered accordingly. But some gaps in the seriation 

 remain obdurate : the more we know the sharper they become. 



The main lines of Molluscan classification have long reached a stable 

 condition : the gaps between the main sub-classes have undergone no 

 reduction in the time of any of us here, in spite of an immense outpouring 

 of new species and genera, trimmings and rearrangements of families and 

 orders, recent and fossil, and in spite of a considerable increase in our 

 knowledge of their comparative anatomy and embryology. I take the 

 following scheme from Prof. Naef's recent and admirable revision' of the 

 Morphology of the group (1926), changing it only by omitting a 

 problematic group of ancient cone-shells {Hyolithes, Conularia, &c.), 

 usually classed as Pteropoda, but which Prof. Naef raises to the rank of 

 an order and terms Odontomorpha, apparently to suggest a relationship 

 with Dentaliiim. In brackets I have aclded certain synonyms which may 

 be more familiar than the primary terms actually adopted. 



MOLLUSCA. 

 {Sub-classes and Orders) (Examples) 



1. Amphineuba 



1. Placophora e.g. Chiton 



2. SoLENOGASTRA e.g. Neomenia 



II. CONCHIFERA 



1. Cephalopoda e.g. Nautilus 



2. Heteroneura (==Prorhipidoglo8somorpha) 



i. Gastropoda e.g. Patella 



ii. Scaphopoda (=Solenoconcha) e.g. Dentalium 

 iii. BrvALViA (=Lamellibranchia) e.g. Nucula 



^ Spengel's Ergebniase u. Forlschritte, III, 1913, and VI,2, 1926. 



G 2 



