THE JNAUTILU8. 



regions. New habitats, with new climates, and with changed 

 chemical qualities of new waters, and with new food materials, 

 must disturb the usual and normal lines of descent. A change in 

 the activity of functions of organs, affecting the physiology of the 

 animal must result. Over stimulation of some functions, and de- 

 pressed activity of others, must change the tenor of life, ultimately 

 evolving new shell characters, and minimizing old ones, or even re- 

 ducing them to a rudimentary state all being effected by change 

 of environment. 



The dispersion of species is scarcely affected by mountain ranges, 

 but oceans are potential barriers. Distribution eastward or west- 

 ward is very slow, owing to the fact that the migrations of water 

 fowls and birds, is mainly from north to south and vice verxn. The 

 spawn, fry or seeds being carried in these migrations, causes a great 

 mixing of fauna and flora, ou the lines of migration. 



The paucity of Unionidse west of the 100th meridian is probably 

 due to the fact that since the laying of the cretaceous beds there 

 and the destruction of the once numerous forms of Naiads that 

 swarmed in that region, by the great upheavals of the country- 

 there has not been sufficient time to repopulate. There are signs, 

 however, of adveutive Naiads, even frdm Europe, there. Mari/m-i- 

 tana ([*/) margaritifera L. and Anodonia cygnea L. from Europe, 

 neither of them fully divorced from their Old World progenitors, 

 seem to have somehow got a lodgement in California and Oregon, 

 though Drs. Lea and Gould did not detect it. Mr. Simpson suggests 

 that the Californian A. cygnea is the parent of the "tramp" .1. 

 < j-iliur Lea, found from Southern California to Mexico and Central 

 America, where it resents having relatives in Europe. 



The most common Unios are those most subject to variation, as 

 seen in U. complanatus Sol., whose progeny are clamoring for " sov- 

 erigu rights " and recognition, which some Uniologists grant, and 

 others deny. On the other hand Naiads vigorously resisting varia- 

 tion, such as U. cylindricus Say, and others, have no near relatives, 

 and are generally rare and with very restricted distribution. 



In living plants, secessions from a given and normal type are 

 readily traceable, and in fossil types, floral and fauual, the grada- 

 tions of differences are well marked. " Connecting links " may be 

 absent, when we seek to trace and run down a species, through the 

 long icons of geologic time. Jiut if a long line of visible road be 

 crossed by a chasm, we cannot resist the conviction that the road 

 was once continuous. 



