POWER OF HEREDITY.--The most important fact brought out in the discussion of the possibility of the transmission of acquired characters is the power of heredity. If organisms did not reproduce their own special characteristics, there could be no fixity of form and no order in organic nature. Nevertheless, if there were no change by individual modification or divergence, in whatever way this may be rendered permanent in the race, there could be no evolution. Hence we can say, with Dr. Eimer, "Any one who thus completely renders allegiance to the supremacy of the principles of the unity of the organic world, who rejects everything which contradicts that principle, cannot help admitting that in truth, as I assert, the ultimate origin of the various kinships in the animal and vegetable kingdom is to be traced to individual differences, and that the difference between the former, like the latter, must be essentially determined by external conditions, by the modification of organic growth." The causes of diversity which interfere with the action of heredity may operate, as we have seen, at the moment of conception, or subsequent to conception. The former class of causes is of great importance, in accordance with the principle, laid down by M. Ribot, of the disproportion of effects to causes, and it is essential, therefore, if children are to be well-born, that their parents should be careful that at the moment of procreation they are fitted for the performance of so serious an act. Mr. J. F. Nisbet in his "Marriage and Heredity" (page 126), well observes, "Twins usually bear a closer resemblance to each other than to their brothers and sisters born at a different period; and the reason generally assigned is that they are conceived under precisely similar conditions. If so, it follows that the difference existing between ordinary members of a family is due to their being born at considerable intervals of time and therefore under changed conditions on the part of their parents." SOBRIETY IN THE FATHER.--Especially does it concern the father, who is the most active agent in reproduction, to see that he is then in a fit condition. This is quite apart from the question of the diseased condition of the organism treated of by Dr. Von Krafft-Ebings, and refers to temporary rather than to continuing causes. Sobriety is in this connection of great importance, and, as appears from a passage, already quoted, in Xenophon, was insisted on at the time of procreation, by the ancients.