176 A REVISION OF THE ASTACIDA. 
In other words, the Astaci of Western North America find their closest kin, not in 
their next neighbors, the crayfishes on the eastern side of the Rocky Mountains, nor in 
those of Eastern Asia, but in the Astaci of Europe; the Cambari of Eastern North 
America are most nearly related, not to the crayfishes on the other side of the Rocky 
Mountains, nor to those on the opposite shore of the Atlantic, but to those of the remotest 
district, Eastern Asia. The two areas inhabited by Astacoid forms alternate with two 
areas of Cambaroid forms. 
“Tf the facts had been the other way,” says Huxley,* “and the West American and 
Amoor-Japanese crayfish had changed places, the case would have been intelligible enough. 
The primitive Potamobine stock might then have been supposed to have differentiated 
itself into a western Astacoid and an eastern Cambaroid form; the latter would have 
ascended the American, and the former the Asiatic rivers. As the matter stands, I do 
not see that any plausible explanation can be offered without recourse to suppositions 
respecting a former more direct communication between the mouth of the Amoor and that 
of the North American rivers, in favor of which no definite evidence can be offered at 
present.” 
In order to explain this singular mode of distribution of the Potamobiine, let it be 
supposed that the marine progenitors of the existing crayfishes were differentiated, not 
only into a northern type with the Potamobiine characters and a southern type with the 
Parastacine characters, but that the Potamobiine type was already differentiated into an 
Astacoid form and a Cambaroid form (with some of the essential characters of the modern 
Cambarus), both of which became widely distributed around the globe in the ecean which 
lay to the north of the ancient continents. After their adaptation to life in fresh water, 
both forms would be driven southward by the climatic changes which have occurred 
within comparatively recent geological periods, into all parts of each continent.f The 
same causes, whether similar climatic conditions or other, which have operated in the 
preservation of so many allied forms of plants and animals on the corresponding sides 
of the Eastern and Western continents, would promote the survival of the descendants of 
the one type of crayfish in Eastern North America and Eastern Asia, of the other in West- 
ern North America and Europe. Unfortunately, we have no paleontological evidence 
touching the former distribution of Astacus and Cambarus, the few fossils known being 
too imperfect for the purpose; but the assumption of the former coexistence of Astacus 
and Cambarus in the same area of distribution receives positive support from the fact that a 
blind Cambarns still survives in the subterranean seclusion of the caves of Carniola. (See 
page 42.) Tt will, moreover, be borne in mind, that in other cases of animals and plants 
that exemplify the same peculiarities of distribution with the crayfishes, direct paleeon- 
tological evidence is not wanting to prove the former general distribution of forms now 
restricted to widely separated localities. To instance a remarkable case among the marine 
Crustacea, the peculiar genus Limulus is represented on the eastern coast of North 
America by Z. Polyphemus. No Limuli exist on the Pacific shores of America nor on the 
coasts of Europe, but closely related species inhabit the eastern side of Asia (Japan, Cochin 
China, the Moluccas, ete.). Now, in the lithographic slates of Solenhofen abundant fossil 
Limuli clearly testify to their former existence in the seas of Europe. 
The reader will observe that in this suggestion of a possible explanation of the pecu- 
liar relations existing between the crayfishes of Western North America and of Europe on 
* The Crayfish, p. 334. 
+ That the crayfishes had become fresh-water animals in Tertiary times is shown by the fossils of Idaho 
and Wyoming. 
