634 THE AMERICAN NATURALIST. [Vow. XXXII. 
invertebrate life. This is strongly suggested by the fact that 
a large proportion of the Cambrian classes embrace forms as 
highly specialized as their successors of the present day, so 
that we are compelled to look many ages back of the Cambrian 
for the appearance of their generalized ancestral forms. 
Of the eight branches of phyla, of the animal kingdom, the 
remains of seven, or all except the vertebrates, have been found 
in Cambrian strata. Adopting the kind of statistics employed 
by Prof. H. S. Williams in his admirable Geological Biology, 
but with some changes necessitated by a little different view as 
to the number of classes living at the beginning of the Cam- 
brian period, it appears that 13 out of 26 classes of the 
animal kingdom, occurring in a fossil condition, already existed 
in the Cambrian, and, if we throw out from the vertebrate 
classes those without a solid skeleton (the Enteropneusta or 
Balanoglossus, Tunicates, Amphioxus, and the lampreys), 13 
out of 22. Also, if we exclude the land forms (Arachnida, 
Myriopoda, and insects), 13 out of 19; and then, throwing out 
the five vertebrate classes found in a fossil state, of 14 inverte- 
brate marine classes 13 occur in the Cambrian.! With little 
doubt, flatworms, nemerteans, Nematelminthes, and Gephyrea 
existed then, and probably the representatives of other classes 
of which no traces will ever occur. 
We shall for our present purpose follow the classification of 
the United States Geological Survey and restrict what was 
formerly called the Archzean to the fundamental gneiss and 
crystalline schists of an unknown thickness, and accept the 
Algonkian, as comprising the Huronian and Keweenawan 
formations. We may assume that the first beginnings of life 
took place toward the end of the Archzan, and that the more 
or less rapid differentiation of class types went on during 
Algonkian time. This view is fortified by the statement of 
Walcott that a great orographic movement, followed by long- 
continued erosion, took place between the Archean and 
Algonkian ages, 
1 Should the Polyzoa be traced back to the Cambrian, as it is not at all 
impossible, the fact would remain that every class of marine invertebrates with 
solid parts is represented in the Cambrian. 
