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Prof. S. J. Hick son. 



symmetrical animals. But characters that are rigid and are not, therefore, 

 subject to fluctuations caused by external forces must be genetic, they are 

 characters transmitted as such by heredity. It is on the rigid characters, 

 therefore, rather than on the plastic characters, that we must rely for a 

 scientific basis for the diagnosis of species. But in the groups of radially 

 symmetrical animals with which we have been dealing more particularly in 

 this lecture, it has been shown that there is a larger proportion of plastic 

 characters than in the bilaterally symmetrical animals, and, moreover, that 

 many of the characters that are plastic in one genus may be rigid in another. 

 The first step therefore in a scientific classification of these animals is to find 

 out the degree of plasticity of the characters it is proposed to use for the 

 diagnosis of each genus and species. Until this is done, the classification can 

 only be regarded as provisional. But the practical difficulty that so often 

 occurs in systematic work is that the niimber of specimens available is so 

 small that no reliable estimates can be formed of the plasticity of the 

 characters they exhibit, and the question arises. What should be done with 

 isolated specimens of which the plasticity of the characters cannot be 

 determined ? It seems to me that if a new specimen differs from previously 

 described specimens of a known species only by one character that is known 

 to be very variable in the genus, it should not be regarded as a new species, 

 but be regarded as a plastic variety of the nearest known species, and if it is 

 desired to call attention to some peculiarity of the plastic form, an additional 

 name should be given to it to indicate this peculiarity. In order to avoid 

 the term " variety," wliich is so frequently applied in higher animals and 

 plants to genetic variations, I have used the term " facies '" to signify a 

 variety which is probably purely plastic in character. Thus, in Millepora 

 nlcicornis, I have used the expressions facies ramosa, facies plimta, facies 

 verrucosa, etc., for various forms of growth or surface markings, the peculiar 

 features of which are almost certainly caused by the external environment ; 

 and in the Foraminifer Sporadotrcma cylindricct I have used geographical 

 terms, such as facies providentice, facies amirantioe, etc., for specimens 

 differing from one another in plastic characters that cannot be easily 

 described by a single word. 



To use either of the terms " variety " or sub-species " in these cases 

 would be entirely misleading, for, as they are commonly used with reference 

 to both the higher animals and plants, they imply genetic differences. Thus, 

 an albino mouse or an albino stock is a variety that breeds true. Mammals 

 with long hair, instead of the short hair that is normal for the species, 

 produce offspring that are long-haired, and plants that have cut leaves 

 instead of the entire leaf that is normal for the species produce offspring with 



