IV THE VOYAGE OF H.M.S. CHALLENGER. 



tially accepted on the Continent as well as here. It is to be regretted that we have no 

 better guide, and not much likelihood of any, so long as it is forgotten that the first 

 requirement in any system is simplicity and obviousness. Systems founded on character- 

 istics absent in most living shells, and in all fossil specimens, are opposed to common 

 sense. "Whatever value they may have, they are not practical. 



With great hesitation I have adopted subgenera, reducing a good many accepted 

 genera to that position. The concession is made to the belief entertained by many that 

 subdivisions are useful. 



The whole system of classification, then, which I have adopted here is not one of which 

 I approve, but is perhaps the least objectionable I could find. 



As regards general results, I cannot do better than repeat here the statements 

 enunciated in 1880 (see Prelim. Report, p. 5, Journ. Linn. Soc. Lond., Zool., vol. xv. p. 

 88), which have been largely confirmed by subsequent writers : — 



(1.) Depth is an important condition in connection with Molluscan life. That is to 

 say, there really are shallow and deep water species and genera, though their bathymetric 

 limits are not constant. 



To some this may seem too self-evident and universally accepted a proposition to 

 need statement. Such would indeed have been the case some years ago, but of late 

 deep-sea dredgings have presented facts which have at least necessitated a revisal of 

 many received opinions on this point. The result, it is true, has never really been doubtful, 

 confirmed as it has further been by evidence gathered from other branches of Natural 

 History. At the same time it is desirable to record here the witness on this matter 

 of the Mollusca of the Challenger Expedition. 



(2.) Temperature much more than mere depth seems an important condition in 

 Molluscan life. 



It is needless to speak here of other conditions, such as light, or food, or oxygen ; 

 because, though there are extreme differences in these respects, and though their influence 

 must be very great, still their precise amount and the nature and direction of their effects 

 are too little known to afford foundation for more than guessing. 



Pressure seemed likely to form a very important condition among those which affect 

 animal life ; the enormous figures representing the square-inch amount of that pressure 

 stirred men's imaginations, and their fancies were supported by the fact that rapid 

 transference to the surface from even a moderate depth destroys life ; but these impres- 

 sions were removed by a remembrance of the laws of hydrostatic pressure, and by 

 substituting a gradual for a rapid transference from deep water to the surface. 

 Temperature, however, remains as an undoubtedly important factor. 

 (3.) Great differences in respect of depth and temperature prove barriers to distri- 

 bution, and so, by preventing the indiscriminate commingling of species, determine and 

 preserve distinct geographical provinces. 



