ue of microscopical anatomy
with its endless variety of stains and reagents made it possible for the
tyro to confirm in a day what von Baer and Mueller had taken weeks of
painful endeavour to discover.[386] But the democratisation of morphology
which followed upon the facilitation of its means of research left an
evil heritage of detailed and unintelligent work to counterbalance the
very great and real advances which technical improvements alone rendered
possible.

This period of rapid development, which set in soon after the coming of
evolution and multiplied the concrete facts of morphology an
hundredfold, may for our present purpose be conveniently divided into
two somewhat overlapping periods, of which the second may be said to
begin with the enunciation by Haeckel of his Gastraea theory. Within the
first period fall the evolutionary speculations associated with the
names of Kowalevsky, Dohrn, Semper, and others; the characteristic of
the second period is the preponderating influence exercised upon
phylogenetic speculations by the germ-layer doctrine in its two main
evolutionary developments, the Gastraea and Coelom theories.

In the first period we might again distinguish two main tendencies,
according as speculations were based mainly upon anatomical or mainly
upon embryological considerations, and it so happens that these two
tendencies are very well illustrated by the various theories as to the
origin of Vertebrates which began to appear towards the 'seventies. We
shall accordingly, in this chapter, consider very briefly the history of
the earlier views on the phylogeny of the vertebrate stock.

In the early days, before the other claimants to the dignity of
ancestral form to the Vertebrates--_Balanoglossus_, Nemertines and the
rest--had put in an appearance, there were two main views on the
subject, one upheld by Haeckel, Kowalevsky and others, to the effect
that the proximate ancestor of Vertebrates was a form somewhat
resembling the ascidian tadpole, the other supported principally by
Dohrn and Semper that Vertebrates and Arthropods traced their descent to
a common segmented annelid or pro-annelid ancestor. The former view is
historically prior, and arose directly out of the brilliant
embryological investigations of A. Kowalevsky, who proved himself to be
a worthy successor of the great comparative embryologist Rathke. His
work was indeed a true continuation of Rathke's. It was not directly
inspired by evo