APPENDIX V 269 



Foraminiferae has been discovered, and the unicellular nature of 

 the Infusoria established. To say that all the supposed proto- 

 plasmatic animals have a nucleus is not yet safe, but it must not 

 be forgotten that in many cases the nucleus is discoverable when 

 properly searched for with the aid of nice histological methods, 

 and that those cases where it has not been found as yet are all 

 cases of uncertainty, partly because careful observations have not 

 been made, partly because the objects themselves are too minute. 

 The probability, therefore, is against the separate existence of 

 protoplasm, and is in favour of the universal presence of the 

 nucleus. This view is strengthened by the discovery of the real 

 nature of Bathybius. 



A cell must, therefore, be regarded as the unit of life, and the 

 problem we are considering becomes to determine the general 

 properties and functions of cells. I reason chiefly upon the basis 

 of zoology, that branch of biology which alone I have studied 

 scientifically. The principal peculiarities of cells, as thus deter- 

 mined, I consider to be as follows : 



1. Irritability. When some motion strikes the cell it may 

 simply act mechanically or give rise to peculiar effects which oc- 

 cur only in living matter. Nothing but some mode of motion 

 ever acts as a stimulus. The effect produced by stimuli is a sen- 

 sation. The stimuli may come from the outside or from the in- 

 side of the cell. The ultimate effects of the irritation may be 

 inhibited, that is delayed or prevented by the cell itself. 



2. The power of doing work, or developing in response to a 

 stimulus, or from some other cause, a certain amount of motion 

 or energy. The work done may be mechanical, electrical, calo- 

 rific, or even luminiferous. The power of doing work cannot 

 be sustained indefinitely, hence the phenomena of fatigue or 

 exhaustion, and recovery. 



3. To set free energy by chemical changes ; each cell must be 

 supposed to maintain a vortex by which matter is continually 

 drawn in from the outside, the elements re-combined, and 

 finally in part ejected, while the shape of the vortex or cell is 

 preserved. 



