Evolution and Symmetry in the Order of the Sea-pens. 131 



cut leaves. But, so far as we can judge from circumstantial evidence, the 

 offspring of a plicate M. alcicornis or a verrucose M. alcicornis would only 

 produce plicate or verrucose offspring if the external conditions of their 

 growth were the same as those of the parents. 



The advantage of using some such word as " facies," instead of " variety " 

 or " sub-species," to express these plastic variations in the sedentary forms 

 of animal life is conclusive, but I would specially urge its adoption because 

 of the tendency shown by some systematists, weary perhaps of the number of 

 different forms of their specimens, simply to mention the name of the species 

 to which they refer them without description. In this way a great deal of 

 valuable information about the plasticity of species is withheld. The 

 tendency there is to provide distinct specific names for local environmental 

 forms has undoubtedly led to a great confusion in our system, and this 

 confusion has, in its turn, tended to discourage systematic work in many 

 groups of sedentary animals. New species are founded, and in a few years 

 merged with others. However carefully the descriptions are made, there is 

 little prospect of the system proposed remaining stable for any length of 

 time, and the result is that it is becoming increasingly difficult to get any 

 one to undertake a systematic description of a large collection of these 

 groups. The advantage of the system of proposing new specific names for 

 specimens differing from others only by certain plastic characters, however, 

 must be acknowledged. It has at least placed on record detailed facts 

 regarding these characters, and provided many excellent figures for future 

 investigations. By the system I have suggested of using the species in a 

 more comprehensive way, and describing the " facies " as frankly a local 

 environmental form, there may be, I hope, a prospect of the revival of 

 detailed analysis of collections. 



4. Evol'utioii of thr. Pmnafulacca. 



In the study of the order of the sea-pens, we find a series of forms 

 showing at tlie one end of the scale almost complete radial symmetry, and 

 at the other end a well marked bilateral symmetry. Moreover, as I have 

 shown, it is among the radially symmetrical that we find that overlapping 

 of genera and species, due to the wide range of variation of plastic characters, 

 which makes systemic work so difficult, and among the bilaterally symmetrical 

 forms that we find, on the whole, well defined genera and species. The 

 question arises, tlien, whether this series represents the general outlines of 

 the evolution of the order, i.e. from radial symmetry to bilateral symmetry, 

 or vice versd, or whether a centrally placed form such as Protoptilum repre- 

 sents the most primitive sea-pen from which the radially symmetrical forms 



