322 LECTURES ON BIOLOGY 



only the male children possessed the characteristic horn} 7 skin. 

 This was probably only accidental, but may on the other hand 

 provide an opportunit}' for obtaining information concerning the 

 cause of sex-determination. 



As a rule the species employed for hybridization will differ 

 from each other in more than one point, so that we have to 

 direct our attention to two, three, or even more 'character-pairs.' 

 Such cases are particularly instructive because they let us clearly 

 perceive the independence and arbitrary mutability of the 

 different ' rudiments' (Anlagen], and because they show that the 

 various parental qualities may reappear in the hybrids in any 

 combination. 



Let us make this clear by choosing the simplest possibility, 

 the difference of only two points, a pea variety with a yellow- 

 wrinkled and one with a green-smooth seed. The yellow colour of 

 the seed of one parent and the smooth surface of the seed-shell of 

 the other prove dominant, so that the hybrids of the first generation 

 produce only ' yellow-smooth ' peas. As now the two character- 

 pairs, 'yellow-green' and ' smooth-wrinkled,' split independently 

 of each other, the hybrids form in this case four different kinds 

 of germ-cells with the ' rudiments ' yellow-wrinkled, yellow- 

 smooth, or green-wrinkled, green-smooth. It is not necessary to 

 mention that here again the egg-cells as well as the male germs 

 are each furnished in perfectly equal proportions with one of 

 these four combinations. If the breeding is continued under 

 strict self-fertilization these four different germ-cells may meet 

 in nine new combinations, which, however, as a result of the 

 prevalence of yellow and smooth produce only four differentiated 

 plants bearing either yellow-smooth, yellow-wrinkled, or green- 

 smooth and green- wrinkled seeds. We see, therefore, that the 

 different parental ' rudiments ' can, in fact, come together in 

 every possible combination. If we now consider the frequency 

 of the single characters we obtain again a result which agrees 

 accurately with our calculations, for we find three smooth to 

 each wrinkled and three yellow to each green pea. It is unneces- 

 sary to carry this instance further : I would only mention that 

 already in this second generation a certain percentage of each 

 of the four different plants proves constant and that therefore 

 the characters yellow-smooth and green-wrinkled have met in 

 a fixed combination. 



