452 ADAM SEDGWICK. 
a consideration of the bearing which these facts have upon any 
hypothesis as to the phylogenetic connection of these various 
organs. 
But, before so doing, it will be well to consider the nature of 
the problem which presents itself. It is universally admitted 
that the Craniata have had a common ancestor. The problem 
to be solved is contained in these questions: What was the 
structure and development of the excretory system of that an- 
cestor? How has it been modified to produce the excretory 
organs which we see in Vertebrates now living? 
I am but too well aware how complicated and difficult the 
problem is, and how insufficient are the data we at present 
possess to enable us to solve it. Of the two sources (geology 
and embryology) from which we can hope to obtain these 
data, paleontology can throw no light whatever upon the 
primitive Vertebrate or its ancestors, for the Vertebrates 
have apparently an antiquity greater than that of the oldest 
fossil-bearing rocks; and even if there are in existence fos- 
siliferous rocks bearing the remains of the ancestor of Verte- 
brates (excluding Amphioxus), we can hardly hope, when 
they are found, to obtain any knowledge of the ontogenetic 
development or structure of soft parts, and the light which 
paleontology throws upon the later history is at present difficult 
to use in settling questions of this kind,! so that we are thrown 
almost entirely upon embryology for the facts; but the facts 
which embryology at present supplies us with are quite inade- 
quate to enable us, even approximately, to solve the problem. 
1 In making out the phylogeny of organs which have had an early origin, 
it seems to me that geology can help us in this way (amongst others). 
Those forms which are found in the oldest rocks, and which have existed 
as small isolated groups, very little changed apparently in structure, to the 
present day, probably retain the same method of development now as 
then. By examining the embryology of such living forms we might 
expect to find the development of certain organs different to that in other 
animals belonging to larger living groups. Turning to the Brachiopoda, a 
group of great antiquity, we find a development of the body cavity which 
is shared by but few animals, and which @ priori we regard as the most 
primitive method of development of that organ known. Now, of the 
animals which resemble the Brachiopoda in this respect, Balanoglossus, 
Amphioxus, and Sagitta are soft bodied, and so not found as fossils; but 
their very isolation at the present day, with recard to their relations to 
other groups, suggests that they are survivals of some larger groups, the 
other members of which have undergone so much evolution that their 
relationship is unrecognisable. The other group, Echinodermata, which 
resents this method of development, is found at its greatest development 
in Paleozoic rocks, and has not undergone any very marked changes since 
that time. It seems to me that, by following this line, some very important 
help might be obtained in helping us to decide questions of organ 
phylogeny. 
