o67 
that in these ancient periods, near their points of origin, animals 
found the earth comparatively unoccupied, and were not only able, 
but in fact forced, to migrate in every direction into different habi- 
tats, and to make perpetual efforts to readjust their inherited struc- 
tures to the new requirements demanded by these comparatively 
unoccupied fields. Food and opportunity would have acted, in 
such localities, as stimulants to new efforts for the attainment of 
more perfect adaptation and for changes of structure useful to that 
end. We can neither imagine the effort to change of habitat and 
consequently change of habits, without their cause the primary 
physical stimulant of change in the environment, nor the changes 
of structure, except as results of efforts on the part of the organ- 
ism to meet the physical requirements of the surroundings. That 
this process should end in the production of structures suited to the 
environment is inevitable. With these factors at work, both without 
and within the organism, the evolution of their structures obey a 
physical law which acts amid a thousand disturbing forces perhaps, 
but nevertheless must act with predominating force in one mean path 
or direction, the resultant determined by the environment and the 
inherited structures of the organism. 
One can compare the changes taking place during the whole of 
Paleozoic time with those known to have occurred in certain iso- 
lated cases in more recent times; such, for example, as that of 
Steinheim, where a single species, finding itself in an unoccupied 
field, proceeded with unexampled rapidity to fill it by the evolution 
of new series and many species, all differing from each other, but 
all referable, by intermediate varieties, to the original form—in this 
example, a single species, the well-known Planorbis equiumbttlica- 
tus.* 
The rapid evolution of the entire family of the Arietidz can also 
be used to illustrate this point. This family originates from one 
ancestral species and yet the process is so rapid that eleven distinct 
series and seven genera arise, culminate and disappear within the 
limits of a single age of geologic history, the Lower Lias of Europe, 
South America and North America.+ 
There are a number of other well-known cases, which could be 
cited, illustrating the quick evolution of species in locations which 
* “ Genesis of Tertiary Species of Planorbis at Steinheim,’ by Alpheus Hyatt, Memoirs 
50 Year Anniv. Bost. Soc. Nat. Hist. 
+ ‘Genesis of the Arietide,”’ by A. Hyatt, Smithson. Contrib. No. 673, Mem. Mus. 
Comp. Zool, 
