INTR OD UC TION 3 



During the past thirty years the theory of organic descent has 

 been shown, by an overwhelming mass of evidence, to be the only 

 tenable conception of the origin of diverse living forms, however we 

 mav conceive the causes of the process. While the study of general 

 zoology and botany has systematically set forth the results, and in a 

 measure the method, of organic evolution, the study of microscopical 



a 







X 



Fig i._ A portion of the epidermis of a larval salamander {Amblystoma) as seen in slightly 

 oblique horizontal section, enlarged 550 diameters. Most of the cells are polygonal m form, con- 

 tain large nuclei, and are connected by delicate protoplasmic bridges. Above v is a branched, 

 dark pigment-cell that has crept up from the deeper layers and lies between the epidermal ceLs. 

 Three of the latter are undergoing division, the earliest stage {sp,rcme) at a, ^J l-';»«^r/»-'f ("^"^"^ 

 figure in the anaphase) at b, showing the chromosomes, and a final stage {telophase), showing 

 fission of the cell-body, to the right. 



anatomy has shown us the nature of the material on which it has 

 operated, demonstrating that the obvious characters ot plants and 

 animals are but varving expressions of a subtle interior organization 

 common to all. In its broader outlines the nature of this organiza- 

 tion is now accurately determined; and the ''cell-theory," by which 

 it is formulated, is, therefore, no longer of an inferential or hypo- 



