PBESIDENTIAL ADDRESS. 727 



We have tlius several cases in which certain species differ from their near allies 

 in the complete absence of a structure which is, as a rule, one of the most distinctive 

 features of the genera to which they are respectively assigned. Should it be 

 possible to prove that the appearance of the organ in question, whether avicularium, 

 vibraculum, or scutum,^ was of the nature of an allelomoi-phic character, its dis- 

 appearance would be readily intelligible. 



The facts which I have indicated with regard to the so-called CellulariidiB 

 have not hitherto been sufBciently discussed ; but I imagine that most systematists 

 who have considered the question have assumed that the scutum, for instance, has 

 undergone parallel evolution in Caberen, Scrupocellaria, and Menipea, either 

 having been independently evolved in each of the three cases (a most improbable 

 supposition), or having independently undergone a series of regressive changes of 

 precisely similar character in the three genera. 



But it is perhaps in the mode of occurrence of adventitious avicularla that we 

 find the strongest reason for believing in the existence of some form of alternative 

 inheritance. We may indeed go as far as to assert that alternative development 

 does actually take place, whether the explanation of the iacts is given by the 

 Mendelian theory or not. The difference between the pointed and the round 

 avicularia is a very definite one, which — it is no exaggeration to say — may be 

 observed in hundreds of species. When these species are arranged under genera 

 according to the result of a study of the whole of the evidence derived from all 

 the characters that have proved valuable in classification, we find that many 

 genera include some species with one type of avicularium and others with the 

 other type. It should perhaps be pointed out that the validity of many of these 

 genera is a matter on which differences of opinion exist. The subject is un- 

 doubtedly a difficult one, and we are far from having arrived at any certainty 

 with regard to the classification of the Cheilostomata. But it is perfectly 

 certain that we could not utilise the two kinds of avicularia in dividing these 

 Polyzoa into two main series, since there are innumerable cases in which both 

 kinds occur in a single colonj'. This is a fact to which I shall return later. 



We may accordingly maintain that, although much is probably faulty in our 

 present system, we have clear evidence that the same genus may include species 

 which differ in the type of avicularium; and, moreover, that these are not 

 exceptional, but, on the contrary, are of common occurrence. A few instances 

 will make these points clear. 



In the encrusting species and in certain others the avicularia commonly occur, 

 as we have already seen, in a position near the orifice of the zooecium, where they 

 are usually either lateral or suboral. In one of the species with lateral avicularia 

 these appendages may be of the pointed type, while in another they may be 

 rounded ; and the same statement may be made with regard to the suboral 

 avicularia. Within the limits of the same genus we may further notice that 

 certain species have lateral avicularia, while others have suboral avicularia. 

 Here, again, we find the same indifference as to the shape of the rostrum and 

 mandible. 



If we might provisionally suppose that the two kinds of avicularia constituted 

 an allelomorphic pair, represented by Aa, and that the lateral and suboral positions 

 indicated a second allelomorphic pair, Bb, the four combinations, AB, Ab, aB, ab, 

 would be theoretically possible. We might, in other words, have pointed or 

 rounded lateral avicularia, and pointed or rounded suboral avicularia. All these 

 conditions actually occur in such genera as Lepralia and Schizoporella ; and;^in 

 some cases two species which agree in the form of the avicularia but differ in 

 their position, or agree in the position but differ in their form, appear on other 

 grounds to be nearly related, one to the other. 



Other cases may be taken from Retepora, an instance where we may feel our- 

 selves on comparatively secure ground, since there are strong reasons for believing 



' The case of the scutum is less striking than that of the other structures under 

 consideration, since conditions intermediate between full development and complete 

 absence are not uncommon. 



