762 RECORD OF CURRENT RESEARCHES RELATING TO 



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

 to say, there really are shallow and deej) water species and genera, 

 though their bathymetric limits are not absolutely constant. 



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

 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 MoUusca of the ' Challenger ' 

 Expedition. 



2. Temperature, even 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 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 sur- 

 face from even a moderate depth destroys life ; but these impressions 

 were removed by a remembrance of the laws of hydrostatic 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 distri- 

 bution, there seems, in ordinary circumstances, no limit to universality 

 of distribution. 



6. There actually are existing species whose distribution is 

 universal, no barriers having availed against their passage. 



7. Still there is no trace, even in these species, of essential, 

 lasting, and progressive change. 



I do not intend to overpress this point, for I allow that it pre- 

 sents merely negative evidence. I do not assert that there are no 

 species of MoUusca which have essentially, permanently, and pro- 

 gressively changed. I only say there are some, even many, which 

 have not done so, that I do not know any which have, and that the 

 burden of proof lies with those who assert the positive. Evolutionists 

 are in the way of saying that a thing being possible is therefore pro- 



