PREFACE TO THE SECOND EDITIOK 



OuE knowledge lias been so mucli increased in extent and exactness 

 in almost every department of Comparative Anatomy since tlie 

 time wlien I converted my " Grundzilge " into the first edition 

 of this smaller manual — the " Grundriss '^ — that the publication 

 of a second edition hardly seemed an easy task. Nevertheless, I 

 gladly undertook it, for I had observed so much new evidence of 

 the importance of the doctrine of development in anatomical 

 enquiry. The road along which science may travel forward success- 

 fully seems indeed to be growing easier, yet the distance which 

 we have made is but short in comparison with that which lies 

 in front of us, and far beyond our view. Every question solved 

 leads again to fresh problems, and renders unstable even what 

 seemed to have taken a definite form. There are, therefore, great 

 difficulties in giving such a comprehensive presentation of the 

 subject as a text-book ought to supply. I have tried as much 

 as possible to evade these difficulties where I have been unable 

 to overcome them. Much remains unaltered, because recent in- 

 vestigations appear to demand fundamental changes, the concrete 

 expression of which cannot be immediately taken in hand. 



I have somewhat modified the arrangement of the matter^ I 

 can hardly be blamed for separating the Brachiopoda from the 

 MoUusca, and treating them as forming an independent phylum. 

 Nor indeed is the change a real one^ for even in my " Grundziige'^ 

 I drew especial attention to the great difference that obtained 

 between them and the " other Mollusca." The Tunicata have 



