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 
ispread 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 ancfent 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 
