296 D'ivrllers of the Sea and Shore 



time that the gigantic reptiles roamed the earth. Over 

 and above this, it appears from these indications that 

 of all portions of the earth's crust, the centers of the 

 great ocean basins have remained the most stable since 

 very ancient times. 



It was once believed that as deep-sea explorations 

 became more extended, the unknown depths of the sea 

 would reveal some survivors of an earlier geological 

 period which would fill some of the gaps in the known 

 pedigree of the animal kingdom. But in only a few 

 instances does this expectation appear to have been 

 realized. The numbers of deep-sea species which have 

 been discovered are very great, yet, generally speaking, 

 they are not unlike the forms that inhabit the shallow 

 waters or else they have adapted themselves to their 

 changed conditions of life by special modifications of 

 their structure. In fact. It is not easy to point out any 

 fundamental feature wherein the deep-sea animals 

 differ from those of shallower waters; what differences 

 do exist are of degree rather than of kind. Therefore, 

 it is thought that because the animals of the deep sea 

 resemble those living in the shallow v/ater at the present 

 time, rather than those of a former age, the abyssal 

 creatures may have migrated to where they now are at 

 some era not long passed, comparatively speaking. Nor 

 has this migration ceased. There are indications that 

 the exodus from our shores, though gradual, is as 

 marked to-day as at any time in the past. 



Yet, notwithstanding that the animals found in the 

 deep sea are as a whole not of a primitive type, certain 

 ancient crustaceans seem to have been harbored there 

 since times remote, or, what is more likely, they sought 



