In all these cases it is probably not always quite new paths which are established in the brain, but use is made of particular tracks in the innumerable nerve-paths already existing in the nerve-cells (neurons) which are 'more thoroughly trodden' by practice, so that the distribution of the nerve-current takes place more easily along them than along others[13]. This much-used metaphor does not indicate the actual structural changes which have taken place, but it serves at least to indicate that we have to do with material changes in the ultimate living elements of the nerve-substance (nerve-biophors) whether these changes be in position or in quality. Now, if such brain-structures and mechanisms acquired through exercise in the individual life could be transmitted, new instincts would certainly arise in this way, and many naturalists hold this view still. [13] This, however, is by no means intended to cast doubt on the possibility that quite new paths may arise during the individual life, as is made probable by the recent investigations of Apáthy, Bethe, and others. If the inheritance of acquired characters had already been proved in other ways, we could not refuse to admit that it might play a part in the higher animals in the modification and new formation of instincts. We should then have to admit that habits can be inherited, and that instincts actually are or may be, as they have often been said to be, inherited habits. But to make the converse conclusion, and to infer from the result of the brain-exercise in the individual life and its similarity to inborn instincts that the latter also depend on inherited exercise, and that there must therefore be inheritance of acquired characters, is hardly admissible. It might be all very well if there were no other explanation! But as instincts depend on material brain-mechanisms which are variable, like every other part of the body, and as, furthermore, they are essential to the existence of the species, and, down to the minutest detail, are adapted to the circumstances of life, there is no obstacle in the way of referring their origin and transformation to processes of selection.