46 BIOLOGICAL SOCIETY OF WASHINGTON. 



none of the marine animals having aqueous respiration, which may- 

 have been thus inclosed by the rising land, would have survived in 

 any instance. The comparatively few forms that did survive in 

 fresh waters doubtless had much time in which to conform to their 

 gradually effected new conditions. 



It is probable that air-breathing mollusks have all been originally- 

 derived from those of aqueous respiration ; and that this important 

 change, and a consequent necessary change of habitat, has been ac- 

 complished mainly by a process of natural selection which was volun- 

 tary on their part, as compared with that by which the gill-bearing 

 mollusks are assumed to have survived the change from salt to fresh 

 waters. Again, that the air-breathing mollusks are confined to the 

 class Gasteropoda, while other mollusks have evidently had equally 

 good opportunities to become air breathers, is a significant fact, 

 but one which, like that of the origin of the air breathers, I cannot 

 discuss at this time. 



While we may not doubt that the whole of the existing life of 

 the globe has come down from former geological periods in un- 

 broken genetic lines, the fact has not been demonstrated by tangi- 

 ble evidence ; and it is well to consider briefly some of the causes 

 of the imperfection of the geological record in that respect. By 

 an investigation of this subject, we shall find that, while a multitude 

 of such lines have certainly terminated at various periods before 

 reaching the present time, it is not necessary to infer that any of 

 them have been imperfect simply because we have not found the 

 proof of their continuity in the shape of fossil remains. The lack 

 of such proof is due to various causes. For example, only the 

 hard parts of animals are capable of fossilization, and many animals 

 have no hard parts. The greater part of the fossiliferous strata of 

 the earth, which now exist as such, are not, and never can be, 

 accessible to human investigation ; and a vast amount of fossilif- 

 erous rocks, now classed as Azoic, may have once contained abun- 

 dant remains of animal life, but which have become completely 

 obliterated by metamorphism or other causes. 



