320 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
spoken of in general as speculation (ancient), observation and in- 
ference (medieval), and experimentation (modern). 
(1) Speculation—tThe idea of organic evolution is as old as our 
record of men’s thoughts, for all the old mythologies are full of it. 
No modern man, therefore, is responsible for the idea, although it is 
a common misconception to load this responsibility upon certain dis- 
tinguished modern students of evolution. For example, the name of 
Darwin isso conspicuous in connection with evolution that many seem 
to think that Darwinism and evolution are synonymous. Until 1790, 
however, organic evolution was a pure speculation, with no basis of 
scientific work. It should be emphasized that the idea of evolution 
has always been present in the mind of man. 
During the latter part of this ancient period of speculation, certain 
facts began to be observed that made some thinking men conclude 
that evolution might be a fact, and not merely a speculation. It will 
be helpful to note briefly, in historical succession, the kind of facts 
that set these men to thinking, and that resulted in the second period 
in the history of evolution, when it became a science. 
In classifying plants and animals, which was the initial phase of 
biology, men rigidly defined the different species, the thought being 
that the different kinds had descended in unbroken succession “ from 
the beginning,” whenever that may have been. When more extensive 
observations were made in the field, numerous intergrades began to 
be found. The species as defined seemed to intergrade freely. In 
other words, the pigeon-hole arrangement, with rigid partitions, did 
not express the facts. It became evident that species had been de- 
fined by man rather than by nature. Some were distinct enough, but 
many intergraded. This intergrading suggested that one species 
might come from another, the intergrades marking the trail. 
The next observations suggesting that evolution might be a fact had 
to do with what was called the “ power of adaptation,” which we now 
call “responses.” It was observed that plants and animals respond 
to changes in environment, often in a striking way. I have seen what 
were regarded as two good species changed into one another by 
changing from a moist habitat to a dry one, or the reverse. This 
ability to respond to changing conditions seemed to indicate that 
species are not so rigid and invariable as had been supposed. 
As technique developed, and the internal structures of plants and 
animals became known, it often happened that rudimentary struc- 
tures were found, which never developed to a functioning stage, but 
which occurred fully developed in related forms. For example, 
it was found that in the developing parrot a set of embryo teeth 
begins, but never matures. The inference was natural that these 
structures had been functional in the ancestors, but had been aban- 
