On page 45 Dr. Gladden, again speaking of the Pentateuch, says:

But the story of Genesis goes back to a remote antiquity. The
last event related in that book occurred four hundred years
before Moses was born; it was as distant from him as the
discovery of America by Columbus is from us; and other portions
of the narrative, such as the stories of the Flood and the
Creation, stretch back into the shadows of the age which
precedes history. Neither Moses nor any one living in his
day could have given us these reports from his own knowledge.
Whoever wrote this must have obtained his materials in one of
three ways:
1. They might have been given to him by divine revelation
from God.
2. He might have gathered them up from oral tradition, from
stories, folklore, transmitted from mouth to mouth, and
so preserved from generation to generation.
3. He might have found them in written documents existing at
the time of his writing.

As many of the laws and incidents in the books of Moses were known to the Chaldeans, the "direct revelation of God" theory is not plausible. On this point Dr. Gladden's opinion supports mine. He says, on page 61:

That such is the fact with respect to the structure of these
ancient writings is now beyond question. And our theory of
inspiration must be adjusted to this fact. Evidently neither
the theory of verbal inspiration, nor the theory of plenary
inspiration, can be made to fit the facts, which a careful study
of the writings themselves brings before us. These writings are
not inspired in the sense which we have commonly given that word.
The verbal theory of inspiration was only tenable while they
were supposed to be the work of a single author. To such a
composite literature no such theory will apply. "To make this
claim," says Professor Ladd, "and yet accept the best ascertained
results of criticism, would compel us to take such positions
as the following: the original authors of each one of the
writings which enter into the composite structure were infallibly
inspired; every one who made any changes in any one of these
fundamental writings was infallibly inspired; every compiler
who put together two or more of these writings was infallibly
inspired, both as to his selections and omissions, and as to any
connecting or explanatory words which he might himself write;
every redactor was infallibly inspired to correct and supplement,
and omit that which was the product of previous infallible
inspirations. Or, perhaps, it might seem more convenient to attach
the claim of a plenary inspiration to the last redactor of all;
but then we should probably have selected of all others the one
least able to bear the weight of such a claim. Think of making
the claim for a plenary inspiration of the Pentateuch in its
present form on the ground of the infallibility of that one of
the scribes who gave it its last touches some time subsequent to
the death of Ezra."

Remember that Dr. Gladden declares, on page 5, that he shall state no conclusions as to the history of the sacred writings which will not be accepted by conservative critics.

On page 54 Dr. Gladden quotes the following from Dr. Perowne:

The first composition of the Pentateuch as a whole could not
have taken place till after the Israelites entered Canaan.
The whole work did not finally assume its present shape till
its revision was undertaken by Ezra after the return from the
Babylonish captivity.

On page 25 Dr. Gladden himself speaks as follows:

The common argument by which Christ is made a witness to the
authenticity and infallible authority of the Old Testament
runs as follows:
Christ quotes Moses as the author of this legislation; therefore
Moses must have written the whole Pentateuch. Moses was an
inspired prophet; therefore all the teaching of the Pentateuch
must be infallible.
The facts are that Jesus nowhere testifies that Moses wrote the
whole of the Pentateuch; and that he nowhere guarantees the
infallibility either of Moses or of the book. On the contrary,
he set aside as inadequate or morally defective, certain laws
which in this book are ascribed to Moses.

So much for the authorship and the inspiration of the first five books of the Bible.