644 ZOOLOGY SECT. 



case, an influence exerted in the opposite direction an influence 

 transmitted from the other parts to the germ-cells has not been 

 proved, and from the nature of the case perhaps cannot be directly 

 proved. Such an influence, it is hardly necessary to add, must 

 be presupposed if we assent to the doctrine of the inheritance of 

 acquired characters. 



Rules or Laws of Heredity. As in the case of the experi- 

 ments referred to on p. 641, by which it has been sought to deter- 

 mine the respective values of nucleus and cytoplasm as bearers 

 of the hereditary qualities, so also in the majority of the experi- 

 ments designed to determine the laws by which the inheritance of 

 paternal and maternal characters is regulated, it has been necessary 

 to have recourse, not to fertilisation between individuals of the 

 same variety, but to crossing between distinct varieties or between 

 distinct species with the production of bastards or mongrels (variety- 

 or species-crosses), since it is only in cases in which the differences 

 between the maternal and paternal characters are strongly marked 

 that it is possible to trace these characters in the descendants. 



It is found that in such experiments in crossing between varieties 

 the result may take one or other of the three following courses. 

 In the first place the progeny may be intermediate in character 

 between the two parents : the paternal and maternal characters, 

 that is to say, may be mixed or blended without any tendency to 

 the predominance of either : a cross between a black and a white 

 variety, for example, in such a form of hybridisation becomes a 

 grey. In a second set of cases, which are the rarest of all, the 

 offspring exhibit paternal characters in one part or set of parts and 

 maternal in another : thus a cross between a black variety and a 

 white yields a piebald or a mottled black-and-white. This is 

 the so-called mosaic form of bastard-inheritance. Lastly there are 

 certain cases in which there is no blending and no mosaic formation, 

 but the paternal and maternal characters remain separate, and 

 appear, or fail to appear, in the progeny in a certain regular order. 

 This is the alternating or Mendelian form of bastard-inheritance. 



It is with this last form of bastard-inheritance that the theory 

 of Mendel, a product of the middle of the last century, but only 

 comparatively recently re-discovered and given due recognition, 

 is concerned. Mendel's original experiments were mainly with 

 varieties of garden-peas. When he crossed two varieties he found 

 that the offspring all presented the characters of one of the parent 

 forms. Thus in the case of a cross between a tall and a short 

 variety whether the short was fertilised by the pollen of the tall 

 or the tall fertilised by the pollen of the short the progeny were 

 tall : in a cross between yellow-seeded and green-seeded varieties 

 the products were all yellow-seeded. Of the two opposed 

 characters, that which reappears in the offspring is known as the 

 dominant, that which does not appear as the recessive character ; 



