From this point of view the question as to the hereditary consequences of external injuries has given rise to great efforts to prove experimentally the truth of this belief, which has existed for centuries. In almost every part of the globe we meet with the assertion that hornless cattle, such, for example, as are bred in South America, or the tailless cats of the Isle of Man, or other domestic animals with similar deficiencies, are descended from a progenitor which lost its horns or its tail through disease or other mishap. Since now, recently, similar assertions have again been put forward to the effect that tailless cats are found among the descendants of feline progenitors who have been robbed of their posterior ornaments by an act of violence, and these cases have been discussed in connection with the pangenesis theory of Darwin, according to which each part of the body is believed to supply material contributions to the germ-plasm, Weismann determined to institute experiments on this point. He started the breeding of white mice whose tails were regularly cut off, without finding as a result, from among 840 young ones derived from such mutilated progenitors, a single one having a malformation or missing tail. However, even this experiment cannot be regarded as an absolute proof, as it at first view might seem, and the negative result was foreseen by the writer of these lines. It is a clear conclusion that if in the case of many vertebrates, for example, salamanders and lizards, as well as in the case of most invertebrates, missing limbs and tails are renewed in the course of their lifetime, it would indeed be very remarkable if their renewal should not take place, at least in the case of the complete rejuvenation of new birth.