No. 381.] A HALF-CENTURY OF EVOLUTION. i 645 
detected below the second Cambrian zone. Even if some pro- 
tocordate Balanoglossus, ascidians, or even Amphioxus had 
already begun their existence in these Precambrian times, they 
could have caused but a little more destruction of life than their ` 
contemporaneous invertebrate allies. As the remains of Ostra- 
codermi and sharks have been detected in Trenton strata, per- 
haps they originated in the Cambrian, when they must have 
been active forces in the elimination of those Precambrian soft- 
bodied animals which connected classes now quite wide apart, 
The rapid increase in the Precambrian population was has- 
tened by the probable fact that this, more than any subsequent 
period, was one of rapid migration and colonization. Vast 
areas of the shallow depths over the site of the embryo conti- 
nent, more or less shut off from the main ocean by shoals, 
reefs, and islands, were, by oscillations of the sea bottom and. 
land, opened up at various times to migrants from the older 
previously settled seas. 
The nature of the Precambrian sediments shows that the 
more open sea bottom was swept by tidal and ocean currents 
varying in strength and extent. The topography of the ocean 
bottom over what is now land must have been more diversified 
than at present. In the late ages of the Algonkian, owing to 
active competition and the struggle for existence in the over- 
stocked areas, the process of segregation or geographical isola- 
tion was rapidly effected, and the migrants from the denser 
centers of growth pressed into the then uninhabited areas 
where, as new, vigorous, and prepotent colonists, they broke 
ground and founded new dynasties. 
At such times as these we can easily imagine that, besides 
the absence of competition, the Lamarckian factors of change 
of surroundings bringing about new habits and thus inducing 
new needs, the use and disuse of organs, together with the 
inheritance of characters acquired during the lifetime of the 
individual, operated then far more rapidly and in a much more 
thoroughgoing way than at any period since, while all through 
this critical, creative period, as soon as there was a sufficient 
diversity in the incipient forms and structures, a selective 
Principle began to operate. 
