170 ANIMALS AND PLANTS VI 



and Schleiden in 1837 and the following years, 

 founded the modern science of histology, or that 

 branch of anatomy which deals with the ultimate 

 visible structure of organisms, as revealed by the 

 microscope ; and, from that day to this, the rapid 

 improvement of methods of investigation, and the 

 energy of a host of accurate observers, have given 

 greater and greater breadth and firmness to 

 Schwann's great generalisation, that a fundamental 

 unity of structure obtains in animals and plants ; 

 and that, however diverse may be the fabrics, or 

 tissues, of which their bodies are composed, all 

 these varied structures result from the meta- 

 morphosis of morphological units (termed cells, in 

 a more general sense than that in which the word 

 " cells " was at first employed), which are not only 

 similar in animals and in plants respectively, but 

 present a close resemblance, when those of animals 

 and those of plants are compared together. 



The contractility which is the fundamental con- 

 dition of locomotion, has not only been discovered 

 to exist far more widely among plants than was 

 formerly imagined ; but, in plants, the act of con- 

 traction has been found to be accompanied, as Dr. 

 Burden Sanderson's interesting investigations have 

 shown, by a disturbance of the electrical state of 

 the contractile substance, comparable to that 

 which was found by Du Bois Reymond to be a 

 concomitant of the activity of ordinary muscle in 

 animals. 



