ANIMALS AND PLANTS. 163 



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 mod- 

 ern science of histology, or that branch of anatomy 

 which deals with the ultimate visible structure of or- 

 ganisms, as revealed by the microscope ; and, from that 

 day to this, the rapid improvement of methods of inves- 

 tigation, 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 respective- 

 ly, but present a close resemblance, when those of ani- 

 mals and those of plants are compared together. 



The contractility which is the fundamental condi- 

 tion 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- 



