440 . THE AMERICAN NATURALIST. (VoL. XXXIIL. 
We think the main value of the work is its illustration of the worth- 
lessness of exterior coloration in shells to the systematist, and the 
interest of this ornamentation problem as an independent question of 
physiology, or possibly of morphology. 
F. N; BALOH. 
Distribution and Variation. — In the Procès Verbaux Soc. Roy. 
Malacol. de Belgique meeting of February, 1898, is reported an inter- 
esting discussion on “L’Emigration considérée comme facteur de 
l’évolution et de filiation des espèces.” It was apropos of a paper 
by M. Arnold Locard in the Compt. Rend. 1’ Acad. Sci., No. 5, 1898, 
on the area of distribution of the molluscan fauna of the boreal Atlan- 
tic in the deeper waters to the south. Locard pointed out that 
recent explorations had shown that forms littoral or sublittoral in the 
boreal regions have spread southward, into ever-deepening waters, 
from an area approximately of common origin, down the European 
and African shores to the latitude of Guinea, in about 2000 fathoms, 
and down the American shores to the latitude of the Antilles, in about 
800 fathoms. The area of distribution would thus have the form of a 
vast triangle of which the apex would rest in about 50 fathoms, some- 
where north of Iceland, while the base connected the north-tropical 
shores of Africa and America, passing upward from east to west from 
a depth of 2000 fathoms to bne of 800. M. Locard’s idea, M. Van 
den Broek thought, was that migration occurs, speaking largely, in 
opposition to variation. That is, a northern species under pressure 
from a changing environment or from crowding might spread along 
the coast where bathymetric and other conditions would be little 
changed, but the temperature change would be great, or it might 
spread downward where the changes would be just the opposite. In 
either case it would follow the line of least resistance; z.e., that in 
which the required variation would be least. Emigration would 
replace adaptation and prove a factor of stability. 
From this idea M. Van den Broek differs. He points out that 
while deeper water was doubtless the line of least resistance in migra- 
tion, and probably called for less variation than migration along the 
coast would have done (as is indicated, moreover, by the archaic 
facies of abyssal life), yet the cause of migration is uncertain. -It 
may be to escape the competition of more competent rivals, or local 
enemies, or parasites, or it may be in the wake of a migrating food 
supply, and in such cases might not be along the line calling for 
least variation. In the case of the deep-water and arctic Mollusca it. 
