194 THE INDUCTIONS OF BIOLOGY. 



system run throughout the other systems; and in each of 

 these universal systems is present the connective tissue hold- 

 ing their parts in position. 



§ 54/. So vast and varied a subject as organic structure, 

 even though the treatment of it is limited to the enuncia- 

 tion of principles, cannot, of course, be dealt with in the space 

 here assigned. Next to nothing has been said about plant- 

 structures, and in setting forth the leading traits of animal- 

 structures the illustrations given have been mostly taken from 

 highly-developed creatures. In large measure adumbration 

 rather than exposition is the descriptive word to be applied. 



Nevertheless the reader may carry away certain truths 

 which, exemplified in a few cases, are exemplified more or 

 less fully in all cases. There is the fundamental fact that 

 the plants and animals with which we are familiar — Meta- 

 phyta and Metazoa — are formed by the aggregation of units 

 homologous with Protozoa. These units, often conspicuously 

 showing their homology in early embryonic stages, continue 

 some of them to show it throughout the lives of the highest 

 type of Metazoa, which contain billions of units carrying on 

 a protozoon life. Of the protoplasts not thus active the 

 great mass, comparatively little transformed in low organ- 

 isms, become more and more transformed as the ascent to 

 high organisms goes on; so that, undergoing numerous kinds 

 of metamorphoses, they lose all likeness to their free homo- 

 logues, both in shape and composition. The cell-contained 

 protoplasts thus variously changed are fused together into 

 tissues in which their individualities are practically lost ; but 

 they nevertheless remain connected throughout by permeable 

 strands of protoplasm. Arising by complication of the outer 

 and inner layers of the embryo and growing more unlike 

 as their units become more' obscured, these tissues are formed 

 into systems, which develop into sets of organs. Some of 

 the resulting structures are localized and special but others 

 are everywhere interfused. 



