462 PROGRESS OF SCIENCE IN THE CENTURY. 



be the degenerate descendants of impulsive (or even 

 more complicated) actions i 



There can be no doubt that a healthy intact frog 

 or newt controls and selects some of its modes of 

 activity, while it is, to say the least, very difficult 

 to prove that a jelly-fish does so. Yet the jelly-fish 

 has got complex sense-organs and a well-developed, 

 though not very complex, system of nerve-cells. 

 What is it that makes all the difference between frog 

 and jelly-fish ? The answer is given in part by a 

 familiar experiment. " Remove the brain of the 

 frog (an operation which it bears with remarkable 

 impunity), and carefully keep it moist and fed, and 

 for the rest of its life, which may easily be prolonged 

 for a year or eighteen months, we have in our hands 

 a machine which responds infallibly to every stimu- 

 lus, but never makes a move in the absence of an 

 easily recognised provoking cause." * 



But while the above experiment shows that the 

 brain is the seat of control, we require some more 

 precise answer, for the brain has many different 

 parts. And here we are helped by one of the ele- 

 mentary facts of minute anatomy, that while the 

 grey matter (a network of nerve-cells) in the spinal 

 cord and in certain parts of the brain receives sen- 

 sory nerves and gives origin to motor nerves, the grey 

 matter of the surface or cortex of the brain is in a 

 measure apart, acting and being acted upon through 

 the mediation of the other grey matter in the lower 

 parts of the brain or in the spinal cord. It is then 

 in this cortical grey matter that we look for the seat 

 of that power of choice and control that distinguishes 

 the higher animals. 



♦ Dr. A. Hill, Trans. Yict. Inst, XXVL, 1892-93, p. 38. 



