But I think it is more wonderful. There exist demonstrably in the brain thousands upon thousands of nerve-elements, whose activity is a definite and limited one, because each particular visual impression, for instance, only excites to activity certain definite nerve-elements, and can leave memory-pictures in these alone. According to my conception of it, the germ-plasm is quite as complex in its composition, and does not consist of homogeneous elements, but of innumerable different kinds, which are not related to the parts of the complete organism indiscriminately, but only to particular parts. But is it allowable to assume that there are invisible nerve-connexions, not only to every germ-cell, but also within the germ-plasm, to every determinant, like the nerve-paths which lead from the eye to the nerve-cells in the optic-area of the brain? For if it were otherwise, how could we conceive of the modification of an organ--as, for instance, the ear-muscles in Man--communicating itself to the precise determinants of these muscles in the germ-plasm? I have often been met with the reproach that my conception of the composition of the germ-plasm is much too complex--but the complexity of Hering's suggestion seems to me to go a long way beyond mine. Hering's ideas, which are not only ingenious but very stimulating, might be accepted as the first indication of an understanding of the assumed inheritance of functional modifications, if it could be proved that such inheritance is a fact; but, as we have seen, that is not the case. The assumption might be permitted, perhaps, if it could be shown that certain groups of phenomena left no other possibility of explanation open except this assumption, but that also, as far as I can see, is not the case. Of course, others hold a different opinion, but chiefly because they have rejected without much reflection the sole explanation which presents itself for numerous phenomena--I mean the processes which we are about to study under the name of 'germinal selection.' But, in any case, Hering's ideas seem to me very valuable, because they make it apparent that, however much we know of the organism, we only know it in a general way, and that numberless delicate processes go on in it which leave no trace for our microscope, and that we can only recognize the final results of numerous invisible and often, in their subtlety, also unimaginable factors. This ought to be taken to heart, especially by all those who speak of simplicity in reference to the germ-plasm. So much at least is certain: If there were any inheritance of functional modifications, we should have another proof that the germ-plasm is composed of determinants, for without them there could be no possibility that the 'experiences' of an individual organ would be transmitted to the germ in the way that the Lamarckian principle implies. Something, and that something material, must be modified in the germ-plasm if the vigorous use of a group of muscles,