176 A REVISION OF THE ASTACI1>.E. 



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 Eocky 

 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. 



" If the facts had been the other way," says Huxley,* " and the "West American and 

 Amoor-Japauese 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 Potamobiinas, let it be 

 supposed that the marine progenitors of the existing crayfishes were differentiated, not 

 only into a northern type with the Fotamobiine characters and a southern type with the 

 Parastacine characters, but that the rotamobiine 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 ocean 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.! 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 aud Eastern Asia, of the other in West- 

 ern North America and Europe. Unfortunately, we have no paheontological 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 Cambarus still survives in the subterranean seclusion of the caves of Carniola. (See 

 page 42.) It 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 palaeon- 

 tological evidence is nut wanting to prove the former general distribution of forms now 

 restricted to widely separated localities. To instance a remarkable ease among the marine 

 Crustacea, the peculiar genus Limulus is represented on the eastern coast of North 

 America by L. 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, etc.). Now, in the lithographic slates of Solenhofen abundant fossil 

 Limuli clearly testily 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. 



t Thai the crayfishes had bei fresh-water animals in Tertiary times is shown by the fossils of [daho 



and Wyoming. 



