156 ANIMALS AND PLANTS. [LECT. 



second ; and even the first can be retained only in a 

 modified form and subject to exceptions. 



But has the advance of biology simply tended to 

 break down old distinctions, without establishing new 

 ones ? 



With a qualification, to be considered presently, the 

 answer to this question is undoubtedly in the affirma- 

 tive. The famous researches of Schwann and Schlei- 

 den 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 metamorphosis 

 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 com- 

 pared 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 contraction has 

 been found to be accompanied, as Dr. Burdon Sander- 



