CHAPTEE II 

 UNITY AND DIVERSITY IN LIVING BODIES 



LIVING bodies are interesting to the man of science from 

 two points of view which, at first sight, seem contradictory : 

 they strike us by their marvellous diversity ; they astonish 

 us by their remarkable unity. 



Among scholars who choose to dwell in admiration on the 

 differences whicl; separate species, some descriptive natur- 

 alists aim either at the knowledge of forms and their 

 classification (Descriptive Zoology and Botany) or at the 

 comparison of forms with each other (Comparative Anat- 

 omy). 



Others, considering each living being as a mechanism by 

 itself, study the conditions and details of the mechanism's 

 working (Physiology and Pathology of a living species) or 

 apply themselves to bringing together the workings of the 

 various specific mechanisms (Comparative Physiology and 

 Pathology) . 



A last kind of these special researches attends to the 

 genesis of a form or specific mechanism, either in the de- 

 scription of its development beginning with the egg (Embryol- 

 ogy of a living species) or in the comparison of the phenomena 

 of such development in different species (Comparative 

 Embryology). 



By joining the study of species of the present day with 

 that of the fossil remains of species which have disappeared 



