290 HENRY SHALER WILLIAMS 
mixed fauna already occupied the eastern area when the Chouteau 
began, and from the evidence of the distribution of the Marshall 
and Waverly faunas, it is probable that it occupied the whole of 
the marine waters then lying over the interior of North America. 
In conclusion I wish to emphasize a particularly valuable 
point made in the paper, viz., the connection between a new 
fauna and the sinking of the land. The theory of my Cuboides 
zone paper required some such hypothesis as this to account for 
the sudden incursion of the general western and northwestern 
Devonian fauna over the New York area. Mr. Weller has sug- 
gested a reasonable solution of the problem. But there is a still 
further inference to be drawn from this and similar facts. May 
not the occupation by the ocean of recently depressed land and 
the changed conditions of environment thus brought about, be 
a fertile and general cause in the modification of the faunas? 
From the facts already known, the inference seems quite prob- 
able that the initiation of new faunas, containing new genera, 
as well as new species, which is observed on tracing the succes- 
sion of formations upwards in time, is intimately associated 
with the occupation by the seas and their contained organisms 
of recently depressed land surfaces.‘. Such radical modification 
of the conditions of environments as would thus take place fur- 
nishes a reasonable condition for the special activity in evolutional 
processes, which is indicated by the sharply distinct character of 
the faunas immediately following an unconformity such as is often 
noticed. The selective effects of migration from the midst of a 
general and adjusted fauna into new conditions of environment 
will undoubtedly account for some of the faunal changes which 
were taking place throughout geological time; but nowhere in 
a series of continuously forming strata is found such definiteness 
of grouping of the species of a fauna as after an unconformity, 
indicating depression of the land after a period of elevation and 
erosion. HENRY SHALER WILLIAMS. 
New HAVEN, Conn., March 2, 1896. 
tSee Ortmann’s discussion of isolation as a factor in evolution. Gvrundztige der 
marinen Tiergeographie, June 1896. 
