128 



Prof. S. J. Hickson. 



are many practical difficulties in the investigation of a large number of 

 examples of the soft and perishable tissues of these organisms, difficulties which 

 are insuperable in the case of the extinct genera ; but it seems improbable 

 that a well-marked discontinuity in the structure of the mesenteries, 

 tentacles, body wall and other parts of the zooids would leave no corre- 

 sponding impressions on the hard parts as they are built up. 



A thorough investigation of the soft parts is, however, most desirable, 

 whenever it is possible, to test the accuracy of the conclusions derived from 

 the study of the hard parts, and we have already a valuable contribution to 

 our knowledge in this direction in the investigations of Mr. Matthai on 

 the anatomy of certain Astraeid corals. Mr. Matthai* gives a list of 10 

 different characters of the hard parts of these corals which are usually 

 regarded* by systematists as of value in their schemes of classifications and 

 comes to the conclusion that none of them have any constant value "and 

 therefore the distinctions based upon them would be arbitrary," but he finds 

 that " any species whose limits had once been settled by the study of both 

 polyps and hard parts can be recognised later from the hard parts alone." 

 With the admitted variability of the hard parts, it is not clear how this 

 recognition of the species can be made : but it does not appear to me that the 

 author of this valuable contribution to our knowledge has been able to prove 

 from his investigations that the soft parts of these corals are less variable than 

 the hard parts. 



In an examination I made some years ago of both the hard and soft 

 parts of a large number of specimens of the genus Millepora, obtained from 

 many different localities both in the East and in the West Indies, If found 

 that the soft parts gave no assistance in the determination of species. But 

 what may be true of one genus or family may not be true of another, and 

 it is qiiite possible that in some kinds of sedentary animals, and more 

 particularly in some forms of floating and drifting animals, there may be 

 true genetic species. The causes that have brought about, in the course of 

 evolution, the discontinuity which, in the bilaterally symmetrical animals, 

 enables us to rec'ognise distinct species are so numerous and involved that 

 it would be presumptuous to assert that they can never affect animals that 

 do not move about by their own muscular effort. In fact the existence of 

 specific groups in the higher plants, every bit as well defined as in the higher 

 animals, should be sufficient to convince any one that causes leading to 

 discontinuity may affect all kinds of sedentary organisms. In the course of 

 my systematic work on Coelenterata, I have come across several instances 



* Matthai, 'Trans. Linn. Soc.,' vol. 17, p. 1 (1914). 

 t Hickson, ' Proc. Zool. Soc.,' 1898. 



