220 ORIGIN OF LIFE IN AMERICA 



It is to south-western North America, therefore, that we 

 must look for the original home of the ancestral group of 

 Potamobius. They still inhabit that area, and may have 

 jspread from there northward to Alaska, and even further 

 to north-eastern Asia, eventually giving rise to Cambaroides. 

 Taking these and many other remarkable facts of dis- 

 tribution into consideration, it appears to me quite pos- 

 sible that the presence of the crayfish Potamobius in 

 Europe and North America, and its occurrence in the 

 western parts only of the latter continent, may be due to 

 an ancient land connection which, as already suggested, 

 joined western Europe and Mexico by way of the West Indies. 

 Whether the family originated in North America or in Europe 

 will have to form the subject for future researches. That this 

 migration took place in very remote times, is implied by the 

 fact that Cambarus primaevus (which Dr. Faxon believes to 

 be a Potamobius), occurs in the Eocene beds of western 

 Wyoming. If such a land bridge as that alluded to actually 

 existed in early Tertiary or late Mesozoic times, it may be 

 asked why do we not meet with any members of the genus 

 Potamobius in the streams of the West Indian islands ? To 

 this we may answer that geologists are practically agreed 

 that in post-Eocene, or even during Eocene times, the whole 

 area of the Antilles was greatly submerged, so that we may 

 suppose that the ancient fauna that wandered across that area 

 from either Europe or North America was largely extermi- 

 nated. That the islands were subsequently again connected 

 with the mainland we may assume from the presence of 

 Cambarus cubensis, a crayfish peculiar to the island of Cuba. 



My views as to the nature and extent of that mid-Atlantic 

 land bridge will be more fully explained in the chapter dealing 

 with the West Indies. The presence or absence of such a land 

 connection, however, is of such vital importance to the eluci- 

 dation of the phenomena of distribution, that I may be ex- 

 cused for quoting still further examples of animals whose 

 range throws light on the solution of this problem. 



In the beginning of this chapter (p. 205) I mentioned the 

 fact that the spade-foot toads (Scaphiopus) have their head- 

 quarters in Mexico and the south-western States of North 

 America, and that their nearest relations are the members 



