OF DIFFERENCE IN STRUCTURE. 17 



in which these things resemble one another that soon 

 it becomes difficult for him to believe that, dijfferent 

 as all are from stones and inanimate objects of every 

 kind, they are not very closely related to one another ; 

 and belief in this view may be strengthened by de- 

 tailed research with the aid of very refined methods of 

 investigation. The student will undoubtedly discover 

 certain essential points in which all agree, and although 

 the hving things referred to differ from one another 

 so enormously in dimensions, he soon finds out that 

 the elementary parts of which the textures of all 

 are composed differ comparatively slightly from one 

 another even in size, while in general structure and 

 appearance they are really much alike. Nor is 

 any great difference in chemical composition to be 

 demonstrated by chemical analysis. Moreover it can 

 be shown that certain phenomena connected with the 

 increase of tissues are common to them all, and when 

 each is examined at a very early period of its forma- 

 tion, the resemblance between many dissimilar tissues 

 is found to be so close that it might be inferred that all 

 were identical at first, and that the ultimate divergence 

 was due rather to the different circumstances under 

 which each was evolved from the homogeneous than to 

 any inherent peculiarities, properties, or powers of 

 the bioplasm that evolved them. There-is, indeed, a 

 period in the development of every tissue and every 

 living thing known to us when there are actually no 

 structural peculiarities whatever — when the whole or- 

 ganism consists of transparent, structureless, semi- 

 fluid living bioplasm— when it would not be possible 

 to distinguish the growing moving matter which was 

 to evolve the oak from that which was the germ of a 

 vertebrate animal. Nor can any difference be dis- 

 cerned between the bioplasm matter of the lowest, 

 simplest, epithelial scale of man's organism and that 

 from which the nerve-cells of his brain are to be 

 evolved. Neither by studying bioplasm under the 



