he retort is obvious. In
his sweeping and poetic vision the paleontologist may fail completely to
find out the nature of the pigments that have gone into the painting of his
picture, and he may confuse a familiarity with the different views he has
enjoyed of the canvas with a knowledge of how the painting is being done.

My good friend the paleontologist is in greater danger than he realizes,
when he leaves descriptions and attempts explanation. He has no way to
check up his speculations and it is notorious that the human mind without
control has a bad habit of wandering.

When the modern student of variation and heredity--the geneticist--looks
over the different "continuous" series, from which certain "laws" and
"principles" have been deduced, he is struck by two facts: that the gaps,
in some cases, are enormous as compared with the single changes with which
he is familiar, and (what is more important) that they involve numerous
parts in many ways. The geneticist says to the paleontologist, since you do
not know, and from the nature of your case can never know, whether your
differences are due to one change or to a thousand, you can not with
certainty tell us anything about the hereditary units which have made the
process of evolution possible. And without this knowledge there can be no
understanding of the causes of evolution.

THE FOUR GREAT HISTORICAL SPECULATIONS

Looking backward over the history of the evolution theory we recognize that
during the hundred and odd years that have elapsed since Buffon, there have
been four main lines of _speculation_ concerning evolution. We might call
them the four great cosmogonies or the four modern epics of evolution.

THE ENVIRONMENT

_Geoffroy St. Hilaire_

About the beginning of the last century Geoffroy St. Hilaire, protege, and
in some respects a disciple of Buffon, was interested as to how living
species are related to the animals and plants that had preceded them. He
was familiar with the kind of change that takes place in the embryo if it
is put into new or changed surroundings, and from this knowledge he
concluded that as the surface of the earth slowly changed--as the carbon
dioxide contents in the air altered--as land appeared--and as marine
animals left the water to inhabit it, they or their embryos responded to
the new conditions and those that responded favorably gave rise to new
creations. As the environment changed the fauna and flora changed--change
for