No. 389.) REVIEWS OF RECENT LITERATURE. 441 
certainly appears that temperature was the main factor, and probably 
it was true that changes in bathymetric conditions called for less 
variation than changes of temperature conditions would have done; 
but, even so, migration proves ultimately a great source of variation. 
The gradual nature of changes on the sea-bottom leads to very 
extended distribution, and consequently into areas which are chang- 
ing in very opposite ways; eg., sinking and rising. With such wide 
geographical distribution come extreme differences, eventually, in 
food and other variation factors. Oscillation of the bottom, M. Van 
den Broek thinks, would be particularly effective in breaking up the 
widely dispersed species, and he points out that a species changed 
by a shoaling of the water would not revert to its old form upon the 
waters again deepening, but would undergo a second change, remov- 
ing it yet a step from the ancestral form which lived under similar 
conditions. M. Van den Broek believes widespread movements like 
the one under discussion have taken place repeatedly, and that the 
ancestors of a given fauna are to be looked for, not in the underlying 
strata, but in distant formations representing the same essentials 
of environment. Thus he finds the ancestral forms of the Belgian 
Miocene sands, not in the underlying Oligocene clays, but in the 
older Miocene of North Germany, while the descendants of the 
Belgian Miocene sands he identifies, tracing a northeast to south- 
west migration, accompanied by an increasing salinity and depth, but 
constant temperature, in the fauna of the Coralline Crag of Suffolk ; 
while above this point, in the Red Crag, he sees the extinction or 
profound modification of the forms and an invasion of new boreal 
forms, indicating a great increase of depth by sinking. 
This discussion is suggestive, but it seems clear that M. Van den 
Broek has not settled the question. If his species migrated into deep 
water because it was the line of least resistance in the first place, why 
did they not do so again when the bottom rose? Again, is it not 
true that no matter what the cause of migration may be, that method 
is chosen because it is the one calling for least modification? We 
will suppose that a migration along the shore calls for greater modi- 
fication than migration into deeper water, and that the original habitat 
is unchanging, but that competition is too great or depredation too 
fierce. It seems that the greatest amount of modification would be 
needed before the competition would be successfully met or the 
depredation resisted. If less were required for this than for the 
change of habitat, the difficulty would be thus met. 
The consideration of migration as a factor of stability is of some 
