180 GENERAL PHYSIOLOGY 



from the parents, not to the next generation, but to the second or the 

 third. This transmission of characteristics to the second or 

 third generation, with omission of the first, is known as 

 reversion, or atavism. Thus, in man it is frequently observed 

 that children have peculiarities of their grandparents which are 

 wanting in their parents throughout life. Indeed, many pecu- 

 liarities, after having remained latent for many generations, can 

 suddenly appear again. This is frequentl}^ observed in domestic 

 animals and cultivated plants which have been artificially bred 

 from the wild forms and been gradually improved. When these 

 are allowed to run wild, as a rule they go back again to the wild 

 state ; every breeder of animals and every gardener is acquainted 

 with many such examples. It would lead too far to discuss these 

 facts in detail, and it would be superfluous, since a great variety 

 of examples have become known through the immortal work 

 of Darwin and the morphological studies that have been carried 

 out in connection with the theory of descent. 



One interesting question in the problem of heredity- has recently 

 come into the fore-ground and has been discussed very actively, 

 namely, the question of the hiheritance of acquired characteristics 

 in multicellular organisms. Are characteristics that have arisen 

 during the individual life through the action of external influences, 

 e.g., mutilations and diseases, inherited, or does inheritance deal 

 with innate characteristics alone, i.e., characteristics that have 

 become established during the germinal development of the 

 organism ? While Darwin ('59), Haeckel ('66), Eimer ('88) 

 and others have defended the view that acquired characteristics 

 are heritable, Weismann ('92, 1) has endeavoured to show in a long 

 series of studies that only those characteristics are inherited the 

 rudiments of which were already present in the germ-cells of the 

 organism. At the first glance it seems surprising that such a 

 question, which apparently is so easy to answer, can be the subject 

 of such opposite views ; for nothing seems simpler than to decide 

 by exjDeriment whether mutilations, performed upon an adult 

 animal, are transmitted to its offspring. In fact, such experiments 

 have been made by Weismann and others. Weismann removed 

 the tails of twelve white mice, of which seven were females and 

 five males, and bred five generations of descendants, a total of 849 

 mice, from these tailless parents, but not a single one was born 

 without a tail ; and in all the adult animals the tails had their 

 normal length. Many such experiments have been performed, 

 but they prove only that in the cases in (question the mutilations 

 are not inherited, and not that no acquired characteristics at all 

 are heritable. Upon the other side a number of examples have 

 been brought forward, from which it would appear that certain 

 acquired peculiarities have been transmitted. But Weismann has 

 subjected all these cases to very careful criticism and has sought 



