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another series of sixty different from the former. And yet in both cases the species are described 
in minute detail. 
The continual experience in investigating a collection of representatives of this genus is 
that, in spite of an initial determination to refrain from adding to the already large number of 
described species, one is forced to do so. And there is no denying that each of these new forms 
has a distinct individuality. This experience inevitably raises a number of aetiological questions 
which it may be useful to state, although they cannot be more than partially answered. Some 
of them at least could only be answered by an investigator having at his command a large 
number of specimens of any given form. 
The outstanding features which present themselves are the following: 
A. That the specific distinctions are all of a relatively trivial sort, such as mode of 
branching, grouping of polyps, length of polyp stalk, strength of supporting bundle, presence 
or absence of a pseudo-crown, the number and the arrangement of the spicules in the anthocodial 
points, the distinction between the spicules in the cortex of the polyparium and those of the 
sterile stalk, and the nature of the spiculation of the canal-walls. 
B. That the species differ one from the other to a large extent in the congeries or 
collocation of such characters as we have mentioned • that is to say, two species with similar 
anthocodial armature may differ in the mode of branching, and vice versa. 
C. That within the limits of a colony there is, in most cases, relatively little variability 
of architecture, though there are, of course, quantitative differences in size of spicules, strength 
of supporting bundle, and the like, which are merely growth characters. We emphasize the 
saving clause “in most cases”, in view of the occurrence of a number of very variable species, 
such as D. giganteci, where there is great variability of anthocodial armature in one and the 
same colony. Later on particular attention is paid to the interesting form referred to; and it 
does not stand alone. 
The multiplicity of form-detail within a narrow range admits of various interpretations: 
I. ( a ) It may be urged that the observed differences are individual, not specific, characters. 
If the observed differences are purely individual, they may be either extrinsic modifications or 
of the nature of intrinsic variations. And before considering these alternatives separately, we 
may note the general fact that in many cases a considerable number of specimens from the 
same or different localities agree together. Thus Ivukenthal speaks of twenty specimens of 
D. sctvignyi , twelve of D. gigantea , ten of D. rubra , seven of D. pumilio : while we have 
examined twenty-five of D. brevirama , twenty of D. ehrenbergi , eighteen of D. gigantea , 
seventeen of D. habereri , and twenty of D. amoebisclera n. sp. 
Therefore the general suggestion that observed differences are purely individual does not 
apply in every case. At the same time, the unsatisfactoriness of basing a new species on a single 
specimen when the specific characters are of such a tenuous nature is plain. 
( b ) It may be said that the observed differences are of the nature of exogenous modifi¬ 
cations correlated with environmental peculiarities. Against a too ready acceptance of this easy 
solution is the occurrence of the same form in widely separated localities. For example, D. robusta 
SI150GA-EXPEDITIE XIII d. 
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