MUSEUM OF COMPARATIVE ZOOLOGY. 183 
or of similar members, the greater the tendency to vary, first, in the 
minor features of these parts or members as compared with each other, 
and, secondly, in the number of similar parts or members in any indi- 
vidual as compared with the average number characteristic of the 
species,* or presented by any other individual of the species. 
What is true of minor details in a complex series — where the com- 
plexity relieves the detail of its importance as a part of the total, con- 
sidered as a subject of selective action —is true of individuals of any 
species, if we suppose the conditions uniform, and of such a kind as to 
bear but lightly on specific characteristics. The latter, in mollusca, are 
chiefly features of external form, color, and sculpture. Now, if the 
form, color, and sculpture are unimportant in the struggle for existence, 
in any given case, it follows that selective action will cease to affect 
them, except so far as they may be indirectly dependent on other 
characters which remain important and continue to be selected. Such 
correlation has not been shown to be frequent in mollusks, if even its 
existence can be said to have been demonstrated. I believe it to be 
an important factor in a certain sort of cases, not however those we are 
considering. The deep sea is doubtless very dark, if not absolutely 
destitute of light. The water must be very quiet, the character of the 
bottom almost uniformly soft and level. Most of the enemies of mol- 
lusks there are blind, or at any rate can have little power of vision for 
objects not luminous. The absence of violent motion in the water 
removes from the category of modifying influences any mechanical ef- 
fects of that medium upon the shell-fish contained in it. So it is evident 
that the factors which would affect the restriction of “tendencies to 
vary” in the above-mentioned characteristics, are almost eliminated 
from the environment, especially if it be compared with that of littoral 
species. The logical result therefore is, that we may expect in the 
deep sea a very wide range of variation in form and sculpture within 
the specific limits of the “flexible” species, and an almost complete 
uniformity over very wide areas of the forms which we may consider 
as “inflexible ” species. 
This is what, according to my judgment, is actually found. With 
* This has been to some extent recognized by Owen in his discussion of vege- 
tative repetition, and is illustrated by the variations of number in the teeth and 
phalanges of cetaceans as compared with seals or other mammals; in the number 
and variation of segments and segmental appendages in worms; in the teeth of 
the Helicide, the coils of the shell in Polygyra, and in the spiny processes of certain 
Muricide among mollusks. See American Naturalist for Sept., 1881, pp. 711, 712. 
