The science of life : fully illustrated in tone and line and including many diagrams, S. 812

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ment of this kind. But, as one would expect from the human analogy, they vary in intensity from case to case, and grade by intermediate stages into slight everyday fluctuations in irritability and inhibitability.

Some dogs proved weaker, more easily upset than their fellows. Corresponding to the two types of nervous breakdown, two tendencies were visible in the normal animals. We have seen in the active, properly adjusted brain a balance between excitation and inhibition ; in some animals the scales are congenitally weighted in favour of one or the other. At one extreme was the excitable animal, in which positive conditioned reflexes could be established readily enough, but inhibitions (e.g. in discrimination experiments) with difficulty, if at all. Such an animal would be naturally confident, inaccurate, and aggressive, and might easily lose control when punished even by a wellknown master. At the other extreme was the inhibitable animal, in which positive reflexes were hard to establish and easily disturbed by distracting stimuli, but in which the inhibitions were strong and ready. This kind of dog is cowardly in general behaviour —shrinking and tucking in its tail at any loud word or sharp gesture.

Between the two are more balanced animals ; but even here an excitable group and an inhibitable one can be distinguished. The dogs of the first are lively and curious, running and sniffing about, almost impossible to keep in order; those of the second are stolid, reserved, sedate. Paradoxically

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enough, it was the first type which was most readily sent to sleep by the monotony of the experimental room; apparently the lively animal needs plenty of stimulation to keep it going.

Pavlov compares his grouping of dogs with the age-old Hippocratic classification of human temperaments. The extreme types are choleric and melancholic; the central types sanguine and phlegmatic. It seems at least that his analysis has the merit of simplicity ; it puts the main facts in order and states them in terms of measurable processes, even if there are many complicating subtleties that for the present at least it leaves unexplained.

But here our account of the dog-mind must end. In 1930 the investigation has been going on for less than thirty years ; and we have summarized the main facts already brought to light. Who can guess what facts and laws will not be added if it continues with the same vigour for another thirty ? We have seen the dog-mind as a very much simpler, more elementary mind than ours, but one with the same basal processes at work in it. Most of the threads of which our daily mental life is wovenlearning and inhibiting, waking and sleeping, alertness and drowsiness, temperamental fluctuations, and even the mysterious disturbances commonly spoken of as “ nerves ’°appear in a more diagrammatic, more readily analysable form in this less elaborate brain.

Let us turn now to the much more difficult problems that we ourselves present.