It is difficult to say where, in the hierarchy of animal progress, the beginnings of intelligence can first be traced. In the articulated animals, such as the insects, spiders, and crustacea, there is abundant evidence of intelligence of a relatively high grade. But even in their case, how hard it is to realize the nature of their experience--to get any adequate notion of their mental processes! We are inevitably forced to describe their psychology in the most general terms. So, too, with forms still lower in the scale of intelligence. Many molluscs unquestionably profit by experience. But can we clearly picture to ourselves the nature and manner of acquisition of this experience? The way in which limpets return to the scars on the rock which form their homes seems to show that they have acquired a practically adequate experience of their near surroundings. Romanes cites[65] some of the earlier observations which have been extended by Professor Ainsworth Davis.[66] I looked into the matter myself some years ago, at Mewps Bay near Lulworth in Dorsetshire. The method adopted[67] was to remove the limpets from the rock, and affix them at various distances from their scars. This can be done without difficulty or injury to the mollusc if one catches them as they are moving. But one must make sure that they are just leaving or returning to their proper homes, and are not taken in the midst of a more extended peregrination, as in that case their special scars cannot be noted. Failure to be careful in this matter vitiated my earlier observations, which are therefore excluded in the following table:-- ---------+------------+------------------------------------+ Number | Distance | Number returned. | removed. | in inches. +-------------+-------------+--------+ | | In 2 tides. | In 4 tides. | Later. | ---------+------------+-------------+-------------+--------+ 25 | 6 | 21 | -- | -- | 21 | 12 | 13 | 5 | -- | 21 | 18 | 10 | 6 | 2 | 36 | 24 | 1 | 1 | 3 | ---------+------------+-------------+-------------+--------+ From the nature of the rock surfaces the removal of a limpet to a distance of two feet almost invariably involved placing them on the further side of an angle. And though some returned over such an angle, the majority did not.