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450 MODERN TIMES.
rendered the theory more correct, especially in regard to
the history of development which CuviER had completely
disregarded.
The scientific treatment of development began at this
time to assume a more conspicuous place than heretofore,
and to exert an influence upon the branches of knowledge
allied to it. PANDER published his original and helpful
investigations upon the development of the chick, proving
that the avian body is formed out of three embryonic layers.
K. E. V. BAER made observations upon the other classes of
the vertebrata, and referred to the different processes of
cleavage in the ovum. The changes of the ovum after impregnation
were observed, and its segmentation described
by others also. Moreover, the development of particular
organs, of the brain, for instance, the eye, the Wolffian
bodies, etc., was made the subject of special study. In this
way attention was drawn to the similarity of embryonic
development in the various kinds of animals. JOHN
HUNTER, KIELMEYER, and afterwards OKEN founded the
theory that the embryos of the more highly-organized,.
animals pass through the stages of development of the
lower classes of animals.
The facts in the history of development associated with
pala^ontological discoveries, which rendered it possible
to recognize the differences existing between fossil plants
and animals and the representatives of their species at
the present day, shook men's faith in the immutability of
form in zoology, and prepared the way for the theory of
descent from. other forms. As early as in the year 1804
LAMARCK declared, in referring to hybridization and the
formation of varieties, that it is only because the judgment
of man is accustomed to deal with small periods of time
that a species is conceived by him to be unchangeable, but
that in reality it does change and adapts itself to external
conditions.
In 1830, LYELL published his Principles of Geology in;
which he contended that to explain the changes in the