The part of the mechanistic theory enunciated by the Neo-Lamarckians seems quite probable. Nature, or environment, does sometimes change organic beings either by chemical or by physical influences, and these changes are not seldom transmitted to the offspring. Antonio Marro (First Eugenic Congress) cites a case where a bull while leaving the stable had its tail cut off, the door suddenly closing; all the calves born through the impregnation of this bull were tailless. Marro also made guinea-pigs epileptic by the resection of the sciatic nerve, and the offspring of the animals were also epileptic. Climate, temperature, moisture, nutrition and unusual activity produce effects upon the organism, and the offspring of the new generation have in their blood and brain the consequences of the habits of their ancestors. Prolonged disuse of an organ causes its degeneration and often its disappearance. High temperature changes the color of insects which is then inheritable or racial. Poisons such as alcohol, syphilis, arthritic diathesis, intoxicants of contagious diseases do also change the germ-plasma. Franz Boas (“The Immigration Commission. Changes in bodily form of descendants of Immigrants”) has found that none of the characteristics of the human types that come to America remain stable. Not even those characteristics of a race which have proved to be most permanent in their old home, as the form of the head, remain the same under the new surroundings. The length of the head of the brachycephalic Hebrews is increased, the width of the head and of the face is decreased. On the other hand, the length of the head of the dolichocephalic Sicilians is decreased, while the width of the head is increased. The effect of these changes is the development of a greater similarity of the descendants of Sicilians and Hebrews, one to the other. The height of body of the American-born Hebrew is increased. This influence of American environment upon the descendants of immigrants shows that acquired characteristics are transmissible. On the other hand, many facts tend to show that acquired characteristics, as a rule, are not transmitted to the offspring. Since the beginning of history circumcision has been practised among the Jews, still the Jewish boy is born with an intact prepuce. For the reason that in the majority of cases acquired characters are not transmissible, the Darwinian rejects the explanation of evolution by appetency, or the use and disuse of certain organs, and assumes a quasi “deus ex machina” in the form of variation.