vi PREFACE 



has proved very great. Relics of a distant past, the 

 features of which have been all but effaced by the 

 passage of time, they preserve for us a record of 

 bygone phases of existence, and often point us to 

 some of the sources from which the modern world 

 of living things has arisen. Although every detail 

 concerning these primitive animals is of interest to 

 the naturalist, in a general exposition it is of prime 

 importance to bring into relief the special features 

 by which these animals help us to follow the tangled 

 thread of phylogeny, and in order to do this satis- 

 factorily it is necessary to compare them with their 

 nearest living and fossil relatives. The chapters in 

 this book are therefore devoted to the comparative 

 study of some of the main groups of the animal king- 

 dom ; a study in which the consideration of some of 

 the most striking instances of primitive animals is 

 involved, though our attention will not be wholly 

 devoted to them. In this way an idea of the com- 

 parative method in morphology (the science of animal 

 structure) and of some of its principal results may 

 be gained, though in the short space available it is 

 impossible to do justice to the great mass of evidence 

 upon which morphological science is based. One of 

 the main objects I have borne in mind has been to 

 distinguish as clearly as possible legitimate from 

 illegitimate morphological speculations, since the 

 failure to distinguish between them has been a 



