Bolton,[226] who has made a systematic study of the memory of children, comes to the familiar conclusion that the scope of memory is measured by the child’s capacity of concentrating its attention. Memory and acute intelligence are not always cognate (the latter proposition, true not for children alone, was known to Aristotle). As a rule girls have better memory than boys (it might also be said that their intelligence is generally greater, so long as no continuous intellectual work, and especially the creation of one’s own ideas, is required). Of figures read only once, children will retain a maximum of six. (Adults, as a rule, also retain no more.) The time of forgetting in general has been excellently schematized by Ebbinghaus. He studied the forgetting of a series of thirteen nonsense syllables, previously learned, in such a way as to be able to measure the time necessary to re-learn what was forgotten. At the end of an hour he needed half the original time, at the end of eight hours two-thirds of that time. Then the process of loss became slower. At the end of twenty-four hours he required a third, at the end of six days a fourth, at the end of a month a clear fifth, of the time required at first. I have tested this in a rough way on various and numerous persons, and invariably found the results to tally. Of course, the measure of time alters with the memory in question, but the relations remain identical, so that one may say approximately how much may be known of any subject at the end of a fixed time, if only one ratio is tested. To criminalists this investigation of Ebbinghaus’ is especially recommended. The conditions of prehensivity of particular instances are too uncertain and individual to permit any general identifications or differentiations. There are certain approximating propositions--e.g., that it is easier to keep in mind rhymed verse than prose, and definite rows and forms than block masses. But, on the one hand, what is here involved is only the ease of memory, not the content of memory, and on the other hand there are too many exceptions--e.g., there are many people who retain prose better than verse. Hence, it is not worth while to go further in the creation of such rules. Forty or fifty years ago, investigations looking toward them had been pursued with pleasure, and they are recorded in the journals of the time. That aged persons have, as is well known, a good memory for what is long past, and a poor one for recent occurrences is not remarkable. It is to be explained by the fact that age seems to be accompanied with a decrease of energy in the brain, so that it no longer assimilates influences, and the imagination becomes dark and the judgment of facts incorrect. Hence, the mistakes are those of apperception of new things,--what has already been perceived is not influenced by this loss of energy.