300 
[May, 
Analyses of Books. 
no more than the ultra-violet. All the luminous rays of the 
spedtrum affedted these animals when employed separately, and 
no difference was recognised between the adtion of diredt sun- 
light, diffused daylight, and light polarised by refledtion. 
The study of eledtrical stimulation leads the author to some 
interesting conclusions. We refer here to the “ summation of 
stimuli,” — a most important physiological principle. If any 
single stimulus, natural or artificial, is given to the excitable 
tissues of a jelly-fish, “ a short period, — the so-called period of 
latency, — elapses, and the animal gives a single weak contradtion. 
If as soon as the tissue has relaxed the stimulation is repeated, 
the period of latency becomes shorter and the contradtion 
stronger. . . . And so on up to nine or ten times, when the 
period of latency is reduced to the minimum and the force of the 
contradtion is raised to its maximum.” Thus successive excita- 
tions arouse the tissue to increased adtivity, and produce in it 
“ a state of greater expedtancy.” Similar effedts have been 
traced in the case of excitable plants, in the heart of the frog, 
and in the reflex adtion of the spinal cord. The author gives 
instances of this summation of stimuli in the organisation of the 
higher animals and in our own, — and others will at once occur 
to the reader. He thinks that even in the domain of psychology 
the adtion of this principle may be traced, whether in “ the 
rhythmical waves of emotion charadteristic of grief,” or in the 
opposite case of the ludicrous. Thus we see from the excitable 
tissues of a plant up to the most complex of our psychological 
processes there is distindt social continuity. 
The experiments described in the next chapter on the “ Sedtion 
of Covered-eyed Medusa” lead to a discussion of Mr. Herbert 
Spencer’s theory on the genesis of nerve-tissue. This theory is 
substantially that “ incipient condudtile tissues or rudimentary 
nerve-fibres are differentiated from the surrounding contradtile 
tissues or homogeneous protoplasm by a process of integration 
which is due simply to use ; so that just, as water continually 
widens and deepens the channel through which it flows, so 
molecular or nervous waves of stimulation, by always flowing 
through the same tissue-tradts, tend ever more and more to exca- 
vate for themselves functionally differentiated lines of passage.” 
Mr. Romanes, in view of the proof that the nerve-plexus in 
question is composed of fully differentiated nerves, thinks that 
this hypothesis still holds as being the only one available, but 
that it “need not now be applied to the genesis of nerve-tissue 
out of comparatively undifferentiated contradtile tissue, but rather 
to the increasing the fundtional adtivity of already well-differenti- 
ated nerve-tissue.” 
Passing over, as compelled by the limited space at our dis- 
posal, the chapters on the sedtion of the naked-eyed Medusae, on 
co-ordination, natural rhythm, and artificial rhythm, we come to 
an investigation on the adtion of poisons. Here we find a 
