496 



NA TURE 



[September 17, 1908 



titular not be precluded from regarding a colony witli 

 avicularia of one type as nearly related to other colonies 

 wliich possess avicularia of the other type ; and we should 

 have some explanation of the fact that many of the genera 

 possess all the different forms of avicularia which are 

 variously distributed among their constituent species. 



1 have so far spoken as if the adventitious avicularia 

 belonged to two types only. This statement requires some 

 further qualification, although it may nevertheless be true 

 that all the forms can be referred to one or other of the 

 .two principal kinds. .'\s a matter of fact, a single Cheilo- 

 stome colony may bear more than two sorts of avicularia ; 

 as, for instance, appendages with large pointed mandibles, 

 in addition to two kinds of those with small rounded 

 mandibles.' This introduces a further complication, about 

 which it is unnecessary to speculate at present. 



It may naturally be asked whether there are any 

 numerical facts which support the suggestions I have 

 made with regard to the significance of the different forms 

 of avicularian appendages. I must admit that the 

 numerical relations are so complicated and apparently so 

 variable that I have not been able to draw any definite 

 conclusion from them. 



E.xperimental evidence is at present wanting, nor would 

 it be easy to devise crucial tests. Even if it were possible 

 to e.xperiment with two colonies of the same species which 

 differ in their avicularian appendages, the result might be 

 negative, since it is not possible to say definitely whether 

 the eggs of a given colony are normally fertilised by the 

 spermatozoa of the same colony or by those of a different 

 colony. Some light may conceivably be obtained from 

 observations on the regenerative processes which may 

 occur in Polyzoa. \ recent Paper by Levinsen ^ gives 

 some information with regard to this point, and there are 

 a few other observations on the same subject scattered 

 through the literature of the Polyzoa. 



It is thus obvious that the speculations in which I have 

 permitted myself to indulge cannot be regarded as more 

 than a guess as to the significance of the causes which 

 underlie the facts observed; but, whether the view I havie 

 outlined has anything to recommend it or not, the observa- 

 tions on which I have depended are, I think, correct. If 

 this be the case, some explanation of the facts is urgently 

 required. The decision of the principles on which the 

 Polyzoa should be classified may not be a matter of 

 immediate practical importance, but our theories of species 

 cannot be regarded as established until they have shown 

 themselves capable of explaining all the cases. Some 

 modification of the Mcndelian theory seems to me to be 

 capable of elucidating the apparently haphazard way in 

 which the several forms of avicularia are distributed in 

 the species of Cheilostomata, and it may perhaps be 

 allowed to afford a working hypothesis that can be used in 

 systematic study. The results of such a hypothesis would, 

 I think, be far-reaching. Whether we are justified in 

 accepting it provisionally or not, I am convinced that we 

 require some hypothesis by which we may regard two 

 specimens as belonging to the same species, even though 

 they may differ in what might at first sight seem to be 

 fundamental respects. And, vice 'oers&., we require the 

 liberty to regard two species as widely separated from 

 each other in the system, even though they possess identical 

 types of avicularia. 



There are other questions which might have been con- 

 sidered in the Cheilostomata, and, in particular, the 

 presence or absence of oral or marginal spines and the 

 forms and distribution of the ovicells. The occurrence of 

 the latter is, however, probably connected with the presence 

 in the young zooecium of tissue which will give rise to an 

 ovary; and this implies the consideration of another factor 

 which is very diflficult to estimate. 



1 In the species nf Retepora, for inst-»nce. there may occur the following 

 types pf -ivicularia, in addition to others that need not be mentioned: 

 Conspicuously large avicularia, some of which are usually fenestral, either 

 pointed (^r) or rounded {b) ; small avicularia, either pointed (t) or rounded, 

 these latter occurring as two well-marked types in which the mandible is 

 respectively longer than broad (rf) or broader than long (e\. The following 

 combmations may occur in individual sp'-cies or colonies; a-\-c^d. a-\-d-^e, 

 a alone, ^4-tr, h-\-d, and others. Examples of some of these combin.ttlons 

 may be seen in Busk's Report on the Polyzoa collected by H.M.S. 

 c//««-nfr,-(Part XXX., 1884). 



2 " Sur la Regeneration totale des Bryozoaires," Acad. Roy. des Sci. de 

 Danemark, Bull, de I'.^nnee 1907, No. 4. 



NO. 2029, VOL. 78] 



I I must not conclude without at any rate referring to 

 the fact that the Polyzoa are by no means the only animals 

 in which dimorphism or polymorphism occurs as the result 

 of blastogenic processes. But among the Coelenterates, for 

 instance, the occurrence of medusoid individuals cannot be 

 considered apart from the question of the sexual cells. 

 Tliere is, however, one series of cases among Hydroids to 

 which allusion may perhaps be made. I refer to the 

 existence of pairs of genera such as Corymorpha and 

 Tubularia, Syncoryne and Coryne, Podocoryne and Hydrac- 

 tinia, in each of which pairs the two genera are dis- 

 tinguished by the fact tliat one produces free medusae while 

 the other has sessile gonophores. There is already some 

 evidence that the validity of these generic distinctions is 

 open to question ; and the free medusoid individual and 

 the sessile gonophore might conceivably be related in such 

 a way as to form members of an allelomorphic pair. The 

 same phylum contains another striking example of 

 dimorphism in the distinction between gastrozooids and 

 dactylozooids in many Hydroids ; while in the Siphono- 

 phora the differentiation of various forms of individual has 

 advanced much further. ' 



But I have already gone much beyond my evidence, and 

 I must bring my remarks to a conclusion by expressing 

 the view that the causes which regulate the differentia- 

 tion of the individuals during the blastogenic development 

 of the Polyzoa are well worthy of further study, and that 

 our knowledge of the unity of the vital processes through- 

 out the animal kingdom gives us reason to believe that 

 they are part of some general Biological law. 



SECTION E. 



GEOGRAPHY. 



Opening Address by Major E. H. Hills, C.M.G., R.E., 

 President of the Section. 



The thirty years that have elapsed since the British Associa- 

 tion last met in this city of Dublin have seen an obvious 

 and rapid progress in the science of geography, and a 

 steady though perhaps not quite so apparent change in 

 the character of that science. 



In 1878 large parts of the earth's surface still reinained 

 untrodden by the feet of a white man ; large areas were 

 open to the enterprise and intrepidity of the explorer ; 

 large spaces were blank paper upon our maps. Now there 

 is but lillle of the earth's surface absolutely unknown. 



It is not my intention to detain you by any recapitula- 

 tion of the work of these years to show you how and by 

 whom these areas have been traversed and the gaps in 

 our maps filled in. I intend rather to speak of the present 

 and of the future work of the geographer, and to do this 

 to any advantage we must at the outset recognise the 

 change that has taken place in the nature of his task, 

 and the fact that the days of individual exploration are 

 over, never to return. We must recognise that sporadic, 

 unorganised effort must be and is being replaced by 

 organised, systematic work, and that the scientific traveller 

 of the last century, with his rough map-making equip- 

 ment, his compass, watch, and sextant has yielded his 

 place to the scientifically equipped survey-party with their 

 steel tapes, theodolites, and plane tables. 



The theme is not a new one to this Section. I find on 

 referring to the transactions of past years that in IQ02. 

 at the Belfast meeting. Sir Thomas Holdich, the President 

 of Section E, said: "We find those spaces within which i 1 

 pioneer exploration can be usefully carried out to be so J 

 rapidly contracting year by year as to force upon our I 

 attention the necessity for adapting our methods for a 

 progressive system of worldwide map-making, not only to 

 the requirements of abstract science but to the utilitarian 

 detnands of commercial and political enterprise." 



These words express succinctly the ideas that I wish to 

 take as the text of my address to-day. I am, however, 

 not ambitious enough to attempt to cover the whole surface 

 of the earth in the brief review that I intend to put before 

 you of the progress of scientific survey. Rather I wish to 

 restrict our outlook to that section of the work in which 

 we may all be considered as having a direct personal 

 interest — namely, the .survey of the British empire, especially 

 those lands under the more immediate tutelage of the 



