The former mode will have had a predominant action where the posterity
of common ancestors constitutes a group of forms standing upon the same
level in essential features, as the whole of the Amphipoda, Crabs, or
Birds. On the other hand we are led to the assumption of the second mode
of progress, when we seek to deduce from a common original form, animals
some of which agree with young states of others.

In the former case the developmental history of the descendants can only
agree with that of their ancestors up to a certain point at which their
courses separate,--as to their structure in the adult state it will
teach us nothing. In the second case the entire development of the
progenitors is also passed through by the descendants, and, therefore,
so far as the production of a species depends upon this second mode of
progress, the historical development of the species will be mirrored in
its developmental history. In the short period of a few weeks or months,
the changing forms of the embryo and larvae will pass before us, a more
or less complete and more or less true picture of the transformations
through which the species, in the course of untold thousands of years,
has struggled up to its present state.

(FIGURES 65 TO 67. Young Tubicolar worms, magnified with the simple lens
about 6 diam.:

FIGURE 65.* Without operculum, Protula-stage. (* Figure 65 is drawn from
memory, as the little animals, which I at first took for young Protulae,
only attracted my attention when I remarked the appearance of the
operculum, which induced me to draw them.)

FIGURE 66. With a barbate opercular peduncle, Filograna-stage;