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 whfch 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 algse 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 tlie more so when the characters presented for 

 its action are salient. For 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, 



