PRESIDENTIAL AUDREHS. 725 



Onychocella, which, alike by their structure and by their early pulaeontological 

 appearance, ni.ay be regarded as representing a primitive type of the Cheilo- 

 stomata. Vicarious avicularia with a rounded mandible occur in certain species 

 which I refer provisionally to Siphojwporella, as well as in a small proportion of 

 the species of Membrmiipora and Flustra. All these may fairly be regarded as 

 belonging to a comparatively undiflereutiated type of Cheilostomata, and their 

 vicarious avicularia are usually the only ones present. It is thus not improbable 

 that the avicularium in these cases really represents an early stage of evolution. 

 But we must notice that precisely similar rounded vicarious avicularia make their 

 appearance occasionally in species of a much more differentiated type, as in the 

 well-known Schizopordla Cecilii ' and in certain other species which may for the 

 present be referred to the same genus. In the majority of the very numerous 

 species of Schizoporella vicarious avicularia are not known to occur, and it is thus 

 impossible to regard them as a typical attribute of the genus. 



The vicarious avicularia, which by their position and general structure are so 

 easily comparable with the ordinary zooecia, are usually supposed to represent an 

 initial stage in the evolution of the avicularium. But if this view be correct, how 

 are we to account for the sporadic way in which these structures occur in a series 

 of genera such as Membranipoi-a, Flustfa, Schizoporella, and Cellepora, the last two 

 of which at any rate are highly specialised in other respects P What conclusion can 

 we draw from the association, in one and the same colony, of this type of avicularium 

 with adventitious avicularia of the most specialised description ? How can we 

 explain the fact that each kind of avicularium occurs in certain species, but not in 

 all the species, of many distinct and not specially related genera ? And, lastly, what 

 is the significance of the fact that certain species of a genus which is normally pro- 

 vided with avicularia may be totally destitute of these organs ? These are some 

 of the problems of which no satisfactory solution has at present been given. On 

 the ordinary view of the way in which the species of a genus are interrelated we 

 should perhaps not expect to find that two species which are closely similar in 

 other respects may be distinguished by possessing entirely different types of 

 avicularia. 



I am aware of the fact that it is perhaps premature to indulge in specula- 

 tions which are unsupported by experimental evidence. But it appears to me 

 worth while to suggest that some of our difficulties might be removed hy 

 appealing to the results obtained by workers on Mendelian inheritance. An 

 essential part of the theory here involved is that in the formation of the gametes 

 of an organism there is a segregation of certain paired or ' allelomorphic ' 

 characters whereby some of the gametes are endowed with qualities by virtue of 

 which they transmit one of the characters, while the rest of the gametes become 

 capable of transmitting the characters of the other member of the allelomorphic 

 pair. It has recently been made probable by Professor Bateaon, whose views 

 have been confirmed by others, that the actual appearance of a particular 

 character may be dependent on a coupling of two allelomorphs belonging to dis- 

 tinct pairs. If only one of them is present the character will not show itself. 

 The phenomenon of reversion on crossing is thus explained as due to the combina- 

 tion of allelomorphs present in the isolated condition in two parental forms. 



Is it not possible that the perplexing occurrence of vicarious avicularia in 

 some of, but not by any means in all, the colonies of certain species may be inter- 

 preted as a reversion due to the combination of two or more allelomorphs that 

 may not have occurred together in the parental forms? We have seen that 

 there is some reason to believe that these avicularia are really of an archaic 

 character, from their occurrence in certain genera of a primitive type, known 

 in some cases by paloeontological evidence to have appeared early in the 

 evolution of the Cheilostwmata. We may further remember that we have 

 distinct evidence that Cheilostomes of a differentiated type may retain certain 

 primitive characters, in the occurrence of a Metnbranipora-like form of ancestrula 

 in 80 many of them. If, then, we may suppose that the appearance of vicarious 



' Kirkpatrick, Ann. Maff. Nat. Hist. (6), v. 1890, p. 21. 



