THE GROUND BETWEEN ANIMALS AND PLANTS. 645 



microscope ; and, fx'oni that day to this, the rapid improvement of 

 methods of investigation and the energy of a host of accurate observ- 

 ers have given greater and greater breadth and firmness to Schwann's 

 great generalization, 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 metamorphoses 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 fundamental resemblance when those 

 of animals and those of plants are compared together. 



The contractility which is the fundamental condition 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 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-Reyniond to be a concomitant of the activity of ordinary muscle 

 in animals. 



Again, I know of no tests by which the reaction of the leaves of the 

 sundew and of other plants to stimuli, so fully and carefully studied 

 by Mr. Darwin, can be distinguished from those acts of contraction 

 following upon stimuli, which are called " reflex " in animals. 



On each lobe of the bilobed leaf of Venus's fly-trap {Dionoea mus- 

 cipula) are three delicate filaments which stand out at right ano-les 

 from the surface of the leaf. Touch one of them with the end of a 

 fine human hair, and the lobes of the leaf instantly close together ' in 

 virtue of an act of contraction of part of their substance, just as the 

 body of a snail contracts into its shell when one of its " horns " is 

 irritated. 



The reflex action of the snail is the result of the presence of a ner- 

 vous system in that animal. A molecular change takes place in the 

 nerve of the tentacle, is propagated to the muscles by which the body 

 is retracted, and, causing them to contract, the act of retraction is 

 brought about. Of course the similarity of the acts does not neces- 

 sarily involve the conclusion that the mechanism by which they are 

 effected is the same; but it suggests a suspicion orf" their identity 

 which needs careful testing. 



The results of recent inquiries into the structure of the nervous 

 system of animals converge toward the conclusion that the nerve- 

 fibres, wliich we have hitherto regarded as ultimate elements of ner- 

 vous tissue, are not such, but are simply the visible aggregations of 

 vastly more attenuated filaments, the diameter of which dwindles 

 down to the limits of our present microscopic vision, greatly as these 

 have been extended by modern improvements of the microscope ; and 

 ' DarwiQ, " Insectivorous Plants," p. 289. 



