GEOGRAPHICAL DISTRIBUTION. 209 



The effect of currents in thus extending the distribution of marine ani- 

 mals would act very differently upon the several classes of the animal king- 

 dom, and its efficiency depends to a great extent upon the nature of their 

 earlier stages, and upon their habits during that period. The time during 

 which the Pluteus of Echini remains helpless at the mercy of the winds and 

 currents is considerable ; from early spring till late in the summer or fall is 

 the usual time required for the full growth of the Pluteus in many species 

 of Sea-urchins, and the distance which the young could thus be transported, 

 even by a sluggish current, during a single season, must be considerable, 

 even under the most unfavorable circumstances. 



As many Echini are not affected by a great range of depth (1200 fathoms), 

 and reproduce themselves frequently at the end of the third year, and even 

 of the second year, after the resorption of the Pluteus, it is not astonishing the 

 same species should be found in every extreme position of depth, latitude, 

 and longitude. 



Various writers have attempted to retrace, in former geological periods, 

 the probable course of the currents and their effect upon the geographical 

 distribution of marine animals ; they all agree in representing, up to the Cre- 

 taceous period, an unbroken equatorial current, passing through Central 

 Asia, Arabia, the northern part of Africa, and connecting with the Pacific by 

 a narrow strait through the Isthmus of Panama. The existence of this con- 

 nection in the Cretaceous period is placed beyond doubt by the presence of 

 an Ananchytes, which I am unable to distinguish from Ananchytes radiata, 

 collected on the Isthmus of Panama, and now in the possession of Yale Col- 

 lege, kindly loaned me for examination by Professor Verrill. From the small 

 number of identical species, either of Mollusca, Crustacea, or Fishes, record- 

 ed on both sides of the Isthmus, this connection must have been very im- 

 perfect at a comparatively recent geological period, — since the existence of 

 the present faunae. 



The question naturally arises. Have we not in the different faunae of both 

 sides of the Isthmus a standard by which to measure the changes which 

 these species have undergone since the raising of the Isthmus of Panama 

 and the isolation of the two fauna? ? If the upheaval of the Isthmus has 

 been gradual, it must, of course, have cut off the deep-water species on both 

 sides of the Isthmus, and gradually have isolated the more shallow, till the 

 littoral species also became separated. As a natural consequence, the deeper 

 we go, the farther back in time we must expect to find the representation, 



