four plans in the process of formation. But his greatest scientific
achievement is perhaps the discovery that all animals originate in eggs,
and that all these eggs are at first identical in substance and structure.
The wonderful and untiring research condensed into this simple statement,
that all animals arise from eggs and that all those eggs are identical in
the beginning, may well excite our admiration. This egg consists of an
outer envelope, the vitelline membrane, containing a fluid more or less
dense, the yolk; within this is a second envelope, the so-called
germinative vesicle, containing a somewhat different and more transparent
fluid, and in the fluid of this second envelope float one or more
so-called germinative specks. At this stage of their growth all eggs are
microsopically small, yet each one has such tenacity of its individual
principle of life that no egg was ever known to swerve from the pattern of
the parent animal that gave it birth.


III.

From the time that Linnæus showed us the necessity of a scientific system
as a framework for the arrangement of scientific facts in Natural History,
the number of divisions adopted by zoölogists and botanists increased
steadily. Not only were families, orders, and classes added to genera and
species, but these were further multiplied by subdivisions of the
different groups. But as the number of divisions increased, they lost in
precise meaning, and it became more and more doubtful how far they were
true to Nature. Moreover, these divisions were not taken in the same sense
by all naturalists: what were called families by some were called orders
by others, while the orders of some were the classes of others, till it
began to be doubted whether these scientific systems had any foundation in
Nature, or signified anything more than that it had pleased Linnæus, for