182 BULLETIN OF THE 
it does not seem to have been realized among naturalists that natural 
selection may act, in certain cases, as successfully by confirming the 
inflexibility of a particular stock, as it does in others by seizing the 
favorable variations of the vast majority of living beings which vary 
indefinitely in all directions. Yet the former method may explain the 
long persistence with but slight modification of certain organic forms 
through immense periods of time and vast areas of distribution. The 
few mollusks which have been recognized as wellnigh world-wide in 
their spread, owe their uniformity, it is likely, to some such cause as 
this. Those mollusks which live on alge and other vegetable matters, 
and are ordinarily called phytophagous, are almost absolutely wanting 
in the depths of the sea, where vegetation except as a sediment from 
near the surface does not exist. We have, then, at the bottom of the 
ocean, a fauna almost exclusively of animal feeders, who receive their 
sustenance chiefly from a constant gentle rain of dead or dying animals 
whose normal existence is passed near the surface of the sea. For this 
reason, the flesh-eaters of the deep sea, among mollusks at least, are not 
obliged to prey upon each other to the same extent as the shallow-water 
forms. The latter have to take part in a fierce struggle for existence, 
among the vicissitudes of tidal and storm waves, variation in elevation 
of land, and a vastly denser population of all sorts. In proportion to 
the whole number, comparatively few of the shells dredged from deep 
water show the drill-holes of enemies of their own kind, or the frac- 
tures and injuries so common in shells from littoral dredgings. 
It will be borne in mind, that the influence of natural selection on 
variations in external characters, the conditions remaining about the 
same, is toward the production of a stable equilibrium in specific charac- 
ters in any species, and the more so when the characters presented for 
its action are salient. or instance, if a few strong, long, sharp spines 
protect a certain species against the attacks of fishes, this character 
tends to be preserved in the species, and as a rule — confirmed by ob- 
servation I may add — there will be little variation in the position and 
number of the spines in question. In another case, where the same 
end has been attained by the production of a profusion of similar spines, 
the presence or absence or exact position of any one or more of the 
spines is less important to the animal, is therefore less sharply restricted 
by natural selection, and the tendency to vary within a certain range is 
less affected, and persists. For these and perhaps other reasons also, it 
may be stated as a general law in animal structures, that the greater 
the number of similar parts in any member of an organic individual, 
