THE SPECIES OF THE GENUS MILLEPORA. 145 



similar to b, or c, or d, or any intermediate or combined form of these 

 varieties. 



By the former course we are practically denying the possibility of 

 considerable plasticity ; by the latter course, while not assuming that 

 it exists, we do not deny it. 



Now the evidence in favour of the view that the Millepores are 

 extremely plastic in their growth increases with every new collection that 

 is examined. Nearly every large specimen shows some branch or plate 

 that is distorted, twisted, compressed, or bent into a different shape 

 from the rest of the coral; its surface shows galls, cups, tubes, wares 

 for the accommodation of crabs, worms, cirripedes, algee, and other 

 so-called parasites. Nor is there any greater constancy of form in the 

 smallest independent specimens that can be found. They may be 

 simply incrusting, or may form a simple crest, or a short pointed 

 process from the base, according to the character of the object on 

 which they grow. It is therefore, in my opinion, not only extremely 

 inconvenient, but positively erroneous, to consider those forms of 

 growth that may be grouped round one " type " as a species distinct 

 from those that can be grouped round another " type." By this plan 

 we either deny the extreme degree of variability which there is reason 

 to believe does occur in nature, or else we employ specific names in a 

 sense altogether different from that in which they are used in the 

 other groups of animals and plants. 



It would be premature to propose to extend my remarks to other 

 genera of corals, but I have already pointed out that there are some 

 reasons for believing that there is not more than one species in the 

 Alcyonarian genus Tubipora and the Hydrocoralline Distichopora. 

 Our knowledge of the soft parts of Madrepora and other genera of 

 Zoantharian corals is so small that it is possible that in the future a 

 very considerable reduction in the species of this genus will also be 

 necessary. Madrepora itself is a genus with a very wide geographical 

 distribution in shallow tropical waters, like Millepora. Its coralla are 

 also subject to extraordinary variability in their form of growth, and 

 the species have been founded on skeletal characters only. All the 

 species, or many of them, may be good, but the classification of the 

 genus must be considered to be unsatisfactory until our knowledge of 

 the anatomy of the polyps of the different varieties has been consider- 

 ably extended. 



