58 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
of organic evolution. The basis of the accepted theory of geological 
evolution is the “uniformitarian doctrine” of Lyell, which assumes 
that the key to the past lies in the present, that the changes that are 
going on today are of the same order and kind as those of the past, 
and, finally, that there is neither beginning nor end to the earth’s 
evolutionary history, but that a slow and orderly development has 
gone on and will continue indefinitely. The proof of this conception 
consists of an array of facts derived from a study of the earth’s crust, 
including its stratified structure, of traces of animal and plant life 
preserved in the rocks, of observed changes in continental contours 
going on today, of erosion going on in coasts and streams, and of a 
considerable array of facts derived from a study of other worlds than 
ours in the making. The theory of geologic evolution meets with 
scarcely any opposition today, although its foundations are no more 
securely based than are those of organic evolution. 
In a sense the proofs of the atomic, ionic, and electron theories 
are even less absolutely established than is that of organic evolution, 
because no one has ever seen nor ever can see an atom, an ion, or an 
electron. Chemical and physical facts are rationalized by assuming 
the existence of these units with their various properties. The only 
evidences of the existence of atoms, ions, and electrons appear in the 
facts that, on the assumption that they exist, the whole array of 
observed chemical and physical phenomena are rationalized and 
bound together into a coherent, consistent, and intelligible system. 
In other words, with the atomic, ionic, and electron theories chemistry 
and physics are highly rational sciences; without these theories the 
phenomena of physics and chemistry would be a hopeless hodgepodge. 
Yet who would say that these fundamental theories are absolutely 
proved ? 
The only type of proof of phenomen’ that cannot be directly 
observed or that pertain to the remote past is circumstantial proof. 
By analogy we conclude that certain changes took place thus and so 
in the past because we observe similar changes going on today. Every 
past event has left a trace, and it is the task of the historian, anti- 
quarian, or evolutionist to discover and to interpret these traces. Some- 
times the traces exist as vestiges in modern life and are meaningless 
unless related to their origin in the past. The task of the student of 
organic evolution is to gather all of the traces of past changes both in 
living creatures today and in the preserved remains of creatures of the 
remote past. A collection of traces of evolution involves many 
