88 EEV. E. BOOG WATSON ON THE 



very heavy task. From the study of this material a few points 

 stand out with special prominence. 



1. Depth is an important condition of molluscan life. That is 

 to say, there really are shallow and deep water species and genera, 

 though their bathymetric limits are not absolutely constant. 



To some this may seem too self-evident and universally ac- 

 cepted a proposition to need statement. Such would have been 

 the case some years ago ; but dredgings from the deep sea have 

 presented facts which demanded a revisal of received opinions on 

 this point ; and while the result in the main cannot be said ever 

 to have been doubtful, and while the evidence of other branches 

 of natural history has already been obtained in this same sense, 

 it is desirable also to record the witness of the Mollusca of the 

 ' Challenger ' expedition. 



2. Temperature, even more than mere depth, seems an im- 

 portant 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 in 

 marine animal life ; the enormous figures representing the square- 

 inch amount of that pressure stirred men's imagination, and their 

 fancies were supported by the fact that rapid transference to the 

 surface from even a moderate depth destroys life ; but these im- 

 pressions were removed by a remembrance of the laws of hydro- 

 static pressure, and by substituting a gradual for a rapid transition 

 from deep water to the surface. Temperature, however, remains 

 as an undoubtedly important factor. 



3. Great differences in these respects of depth and temperature 

 prove barriers to distribution. 



4. Great length of time naturally helps escape from these 

 barriers, for in the lapse of years accidents are likely to occur 

 enabling species to evade difficulties which would in ordinary 

 circumstances prove insurmountable. Hence the finding of a 

 living species in a fossil state will always justify the expectation 

 of its having a wide local distribution. 



5. Where barriers of depth and temperature do not check dis- 

 tribution, there seems, in ordinary circumstances, no limit to 

 universality of distribution. 



