PREFACE TO THE SECOND EDITION. 



IT was originally our intention to publish this work in two 

 parts, the first, which appeared in 1886, being intended as an 

 introduction, while the second was to form the main body of the 

 work and to include the study of a series of type-forms. The 

 pressure of other work, however, delayed the completion of the 

 second part, and meanwhile several laboratory manuals appeared 

 which hi large measure obviated the need of it. Nevertheless 

 the use of the introductory volume by teachers of Biology, 

 and its sale, slowly but steadily increased. It soon appeared, 

 however, that in some cases the work was being employed not 

 merely as an introduction, as its authors intended, but as a 

 complete course in itself; though the wish was often expressed 

 that the number of types were somewhat larger. These facts, 

 and the many obvious defects in the original volume, induced 

 us to undertake the preparation of a second and extended edition. 



With increased experience our ideas have undergone some 

 change. We are as firmly convinced as ever that General Biol- 

 ogy, as an introductory subject, is of the very first importance ; 

 but we are equally persuaded that it must not trespass too far 

 upon the special provinces of Zoology and Botany. The present 

 edition, therefore, differs from the original in these respects: 

 first, while the introduction has been extended so as to in- 

 clude representatives of the unicellular organisms (Amoeba, 

 Infusoria, Protococcus, Yeasts, Bacteria), the publication of a 

 second volume has been abandoned. It is hoped that the work 

 as thus extended may serve a double purpose, viz., either to 

 be used as an introduction to subsequent study in Zoology, Bota- 

 ny, or Physiology ; or as a complete elementary course for 

 general students to whom the minutiae of these more special sub- 

 jects are of less importance than the fundamental facts of vital 

 structure and function. We believe that a sound knowledge of 



