56 ' THE DATA OF BIOLOGY. 



which is initiated at one extremity and accumulates in its 

 course to the other extremity, are questions which cannot 

 yet be answered. All we know is that agencies capable of 

 working molecular changes in nerves are capable of calling 

 forth from them manifestations of activity. And our evi- 

 dence that nerve-force is thus originated, consists not only of 

 such facts as the above, but also of more conclusive facts 

 established by direct experiments on nerves — experiments 

 which show that nerve-force results when the cut end of a 

 nerve is either mechanically irritated, or acted on by some 

 chemical agent, or subject to the galvanic current — experi- 

 ments which prove that nerve-force is generated by whatever 

 disturbs the molecular equilibrium of nerve-substance. 



§ 22. The most important of the re-actions called forth 

 from organisms by surrounding actions, remains to be noticed. 

 To the various forms of insensible motion thus caused, we 

 have to add sensible motion. On the production of this 

 mode of force more especially depends the possibility of all 

 vital phenomena. It is, indeed, usual to regard the power of 

 generating sensible motion as confined to one out of the two 

 organic sub-kingdoms; or, at any rate, as possessed by but 

 few members of the other. On looking closer into the matter, 

 however, we see that plant-life as well as animal-life, is uni- 

 versally accompanied by certain manifestations of this power ; 

 and that plant-life could not otherwise continue. 



Through the humblest, as well as through the highest, 

 vegetal organisms, there are ever going on certain re-distri- 

 butions of matter. In Protophytes the microscope shows us 

 an internal transposition of parts, which, when not imme- 

 diately visible, is proved to exist by the changes of arrange- 

 ment that become manifest in the course of hours and days. 

 In the individual cells of many higher plants, an active 

 movement among the contained granules may be witnessed. 

 And well-developed cryptogams, in common with all phanero- 

 gams^ exhibit this genesis of mechanical motion still more 



