DE. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
807 
It has already been stated that the primary zooid is almost always larger and more 
perfectly developed than the secondary zooids derived from it. If, then, the secondary 
zooid owes its rudimentary structure to inheritance from a slightly more advanced bud, 
may not the primary zooid, as Dr. W. K. Brooks has suggested to me, have been 
produced by the multiplication of a still more perfect bud, like the zooid, for instance, 
of Halisc,ej)trum which possesses a pair of mesenterial filaments X This in turn may 
have been formed by the multiplication of a more highly organised bud, and so on 
until a fully developed polyp stood at the beginning of the series. This will be 
rendered more clear by an illustration, in the consideration of which it is necessary to 
bear clearly in mind the fact that the immature bud of the sexual polyp performs the 
same function as the zooid and that this function is of vital importance to the 
organism. 
Suppose a secondary bud, A, to give rise by asexual multiplication to a tertiary bud, 
B, which remained longer in a rudimentary state and developed less perfectly than A, 
and hence performed more perfectly the function of taking in water. In a succeeding 
generation B gives rise to still more rudimentary individuals, C, and so on through 
many generations until true zooids, permanently rudimentary, were produced. The 
functions of the rudimentary and of the fully developed individuals being entirely 
different, the interme liate or transitional forms would perform both functions less 
perfectly. They would therefore tend to disappear by natural selection until a colony 
would result like Renilla in which no well-marked transitional forms existed. Such 
a process is widely different from direct degeneration since each stage of the series is 
not represented in the preceding stage. Thus in the foregoing illustration C is not 
represented in the preceding stage by B, but is an entirely new individual produced as 
a bud upon B; and this is true of each succeeding stage. If, then, the ancestral 
history of a zooid could be followed backward from one generation to another we 
should not find it becoming more and more highly organised, but a point would be 
reached when it would entirely disappear. 
This view is perhaps of too speculative a nature to be accepted without reserve, 
but it has at least the merit of showing how structures like zooids, of considerable 
complexity, might suddenly arise without direct descent from or the gradual modifica¬ 
tion of any corresponding structures in a preceding generation. 
In regard to the nature of such structures as the zooids, Huxley’s definition of the 
“organs” of the Hydrozoa appears to me most satisfactory. They are, namely:— 
“ Organs which tend more or less completely to become independent existences or 
zooids.” (The term zooid is here used in a general sense and not in the special 
sense employed in this paper.) A careful distinction must, however, be drawn between 
these “ organs ” and those which are due to the direct degeneration or other modifica¬ 
tion of complete individuals; and the possibility must be borne in mind that these 
different kinds of structures may co-exist in the so-called polymorphic communities. 
Beaufort, N.C., August 1 , 1882. 
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