WOLFFIAN DUCT AND BODY IN THE CHICK. 33 



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, palseontology 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 

 palaeontology 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 iu 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 tiiat 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 a 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 regard 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 

 presents this method of development, is found at its greatest development 

 in Palaeozoic 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. 



