EYDROIDS EASTERN CANADA 335 



SESSIONAL PAPER No. 38a 



be excluded. Among the individual trophosome characters the nature of the opercular 

 apparatus is a good character, therefore all other characters should be excluded and 

 the opercular apparatus must form the one and only basis for the whole system of 

 classification. 



Let us examine the argument piece by piece. In the first place, Avithout trying to 

 settle the relative value of individual and colonial characters, are the colonial charac- 

 ters of such little value that they should be neglected entirely in classification? In 

 connection with this, Levinsen drew an analogy in his earlier paper (p. 184) and was 

 so well satisfied with it that he quoted it in his later paper (p. 255). It is this: "A 

 zoological system based on that kind of characters may be compared to a botanical, 

 in which the chief stress was laid on the inflorescences and not on the structure of the 

 flowers. In both cases, the genus would contain a number of heterogeneous species. 

 It can hardly be deemed doubtful that constant differences 'in the structure of the 

 single individuals in question, of the hydrothecsp or hydranths, ought to be preferred 

 as systematic characters, and that colonial characters ought only to be used when 

 structural diversities were not to be found." 



The analogy is somewhat unfortunate as in many cases the inflorescence is char- 

 acteristic not only for the genus but even for the family. What more constant char- 

 acter would it be possible to get than the head of the Compositse, the loose raceme of 

 the Ranunculaceae or the compoiind umbel of the Umbelliferse if In the great majority 

 of cases each species has a typically characteristic habitus and whatever in addition 

 may be used as a basis for first diagnosis, as soon as the plant becomes familiar, it will 

 be recognized by its inflorescence rather than by any single characteristic of the flower 

 itself. So too in the case of the hydroids, each species has its own typical habitus by 

 which it is recognized and if the genus has not so much the worse for the genus or 

 the validity of it. The fact that the habitus of the young colony may be somewhat 

 different to that of the colony at a later period and depends to a certain extent on 

 environment, rather increases than decreases the value of this as a distinctive char- 

 acter when the life history is known. In any case even if the colonial characters, 

 taken as a whole, are not of so much value as the individual characters, there is no 

 reason that they should be discarded. 



Turning to the next part of the argument, the characters of the gonosome are 

 neglected because they are less important than the characters of the trophosome. Are 

 the characters of the gonosome of so little account? Turning again to the floral 

 analogy, how much of any system of classification would be left if all the references 

 to the nature of the gyncecium and androecium and their relations to other parts of the 

 flower were left out ? In all other families of hydroids the characters of the gonosome 

 are used extensively for taxonomy, why should they not be used in the Sertularidae? 



Finally, going back to the floral analogy once more, is it possible to find a single 

 family of plants of any size that is divided into genera on the basis of a single char- 

 acter of the floral envelopes? In the hydroids as well, although one character in a 

 family may be prominent, it is seldom that the paucity of characters is so marked as 

 to make it necessary to rely on one character of the trophosome alone as the deter- 

 mining factor throughout. 



Some of the points as they appear in Levinsen's paper may well be considered. 

 After showing that the different species of Selaginopsis do not fit in in with the generic 

 idea when based on the nature of the opercular apparatus, the following statement is 

 made : " The fact that there is no constant relation between the structure of the zooids 

 and the colonial form, or to express it in another way, that they are incommensurable 

 values defined by different laws, must have the logical sequence, that one of them can- 

 not be substituted for the other, and, therefore, a genus ought never to be instituted 

 solely on the basis of a difference in the colonial form, when otherwise the zooids pre- 

 sent distinct structural diversities " (p. 259). To state that the conclusion that " there 

 is no constant relation between the structure of the zooids and the colonial form " is a 



