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 IlelicidcE, the coils of the shell in Polygyra, and in the spiny processes of certain 

 Muricida among mollusks. See American Naturalist for Sept., 1881, pp. 711, 712. 



