XV THE PHILOSOPHY OF ZOOLOGY 6fi5 



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

 ments i-efeiTc'd to on p. G61, by which it has been sought to 

 determine the respective vakies of nucleus and cytoplasm as 

 beavers of the hereditary qualities, so also in the majority of the 

 experiments designed to determine the laws by which the in- 

 heritance 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 ditferences 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. jMendel'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 white-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 : 

 the varieties presenting these characters are accordingly spoken of 

 respectively as either dominant or recessive. When the hybrids, all 

 showing the dominant character, produce a second generation by 

 self-fertilisation, there is a re-appearance of the recessive character 



