6 THE NAUTILUS. 
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 versa. The 
spawn, fry or seeds being carried in these migrations, causes a great 
mixing of fauna and flora, on the lines of migration. 
The paucity of Unionidee 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 adventive Naiads, even from Europe, there. Margari- 
tana (Unio) margaritifera L. and Anodonta 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” A. 
exilior 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- 
erign rights” and recognition, which some Uniologists grant, and 
others deny. On the other hand Naiads vigorously resisting varia- 
tion, such as U. eylindricus 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 faunal, 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 xeons of geologic time. But if a long line of visible road be 
crossed by a chasm, we cannot resist the conviction that the road 
was once continuous. 
