Evolution and Symmetry in the Order of the Sea-pens. 127 



3. On the Classification of Sedentary Animals. 



The difficulties that are found in separating the radially symmetrical 

 Pennatulacea into defined specific groups are met with even in a more 

 pronounced degree in the stud}'- of the corals, in the sub-orders of Alcyonaria, 

 and in some other groups of sedentary animals ; and it may even be suggested 

 that, as our knowledge of the range of variation increases, the conchision will 

 be reached that, in some cases at least, the evokition of those discontinuous 

 groups which are commonly recognised as " species " does not occur in nature. 

 In some of the genera of sedentary Coelenterata, such as Millepora, Tubipora, 

 and Stylaster, and in some sedentary Foraminifera such as Sporadotrema and 

 Polytrema, with a very wide geographical distribution in shallow water, very 

 careful examination of a large number of specimens and an analysis of all 

 possible characters that would be used in classification show very little if any 

 evidence of the existence of discontinuous specific groups. There are 

 differences between specimens from different localities in the mode of 

 branching, in colour, in the size of the zooids, and in the. arrangement of the 

 zooids on the colonies, but these characters are found to be so variable when a 

 number of specimens are examined from the same locality that it is impossible 

 to use them in the definition of species. Similarly Bernard, in preparing his 

 monumental catalogue of the Madreporaria in the British Museum, found the 

 difficulties of maintaining the old or establishing new species so great that he 

 abandoned the orthodox binomial system and grouped the .specimens according 

 to their growth forms and geographical distribution, " The task of establishing 

 genetic species," he writes, " is practically hopeless in such a case as Porites."* 

 And Wood Jonesf after a careful study of the different forms assumed by 

 the corals on a reef writes, " There is no doubt that a great number of our 

 museum-made species are mere vegetative varieties, produced in response to 

 the demands of the environment." 



N"or are the difficulties of determining species confined to the sedentary 

 Coelenterata, as we see, for example, in the statement made by Gregory! in 

 his introduction to the catalogue of the Jurassic Bryozoa, that he came 

 reluctantly to the conclusion "that there are no true genera among 

 Cyclostomata bat only certain convenient artificial gi'oups of species." It 

 might be urged in reply to these expressions of opinion, and to many others of 

 a similar kind that have not been quoted, that, unless the anatomy of the 

 zooids and their connections with one another are as carefully studied as the 

 skeletal structures, the conclusions are based on insufficient evidence. There 



* Bernard, ' Catalogue of Madreporaria, Brit. Mus.,' V, p. 27 (1905). 

 t Wood Jones, ' Proc. Zool. Soc.,' 1904, p. 555. 



% Gregory, ' Catalogue of Jurassic Biyozoa, Brit. Mus.,' 1896, p. 21. 



