60 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
a state of increased activity, and also of producing 
in it a state of greater expectancy. We have, more- 
over, seen that this effect depends upon the repe- 
tition of the process of stimulation, and not upon 
that of the process of contraction. 
Now, effects very similar to these have been 
found to occur in the case of the excitable plants 
by Dr. Burdon-Sanderson; in the case of the frog’s 
heart by Dr. Bowditch; and in the case of reflex 
action of the spinal cord by Dr. Stirling. Indeed, 
the only difference in this respect between these 
four tissues, so widely separated from one another 
in the biological scale, consists in the time which 
may be allowed to elapse between the occurrence 
of the successive stimuli, in order to produce this 
so-called summating effect of one stimulus upon 
its successor: the memory, so to speak, of the 
heart-tissue for the occurrence of a former stimulus 
being longer than the memory of the jelly-fish 
tissue; while the memory of the latter is longer 
than that of the plant tissue. And I may here 
add that even in our own organization we may 
often observe the action of this principle of the 
summation of stimuli. For instance, we can 
tolerate for a time the irritation caused by a crumb 
in the larynx, but very rapidly the sense of irrita- 
tion accumulates to a point at which it becomes 
impossible to avoid coughing. And similarly with 
tickling generally, the convulsive reflex movements 
to which it gives rise become more and more incon- 
trollable the longer the stimulation is continued, 
until they reach a maximum point, where, in 
