320 LECTURES ON BIOLOGY 



the fertilized egg-cell, the plants developed from them must 

 naturally vary greatly. If a plant contains only the red, or 

 only the white nuclear loops, it can naturally only produce red 

 or white flowers and must under self-fertilization keep pure in 

 all future generations ; but in the case in which the two united 

 germ-cells were differently supplied, where one possessed the 

 paternal, the other the maternal characters, intermediate hybrids 

 with pink flowers will originate, and these will always again 

 produce two kinds of germ-cells, and, therefore, even under self- 

 fertilization, always in exactly the same manner ' split ' into three 

 kinds of descendants exactly as the hybrids of the first generation. 

 A simple calculation shows that the ' split ' in perfect accord 

 with Mendel's results must occur in the proportion of 1 : 1 : 2, 

 in other words, the hybrids will always produce 25 per cent, red- 

 flowering constant plants, 25 per cent, white-flowering constant 

 plants, and 50 per cent, intermediate ' mendeling ' plants. 



These results are corroborated by numerous crossing experi- 

 ments made with animals. The well-known Helix hortensis 

 and Helix nemoralis occur in two varieties, of which one has 

 a shell with a delicate dark ribbon design, while the shell 

 of the other variety is without this ornament. If we pair a 

 ' ribbon snail ' with a ' ribbonless snail ' we shall obtain only 

 ribbonless descendants. But as Arnold Lang was able to show 

 by the most painstaking experiments extending over several 

 years, the suppressed ribbon design reappears in the second 

 hybrid generation in about 20 per cent, of all the descendants, 

 and remains 'thence constant in all subsequent generations. 

 The behaviour of the snail is therefore exactly analogous to that 

 of peas with red and white flowers, or the nettles with dentate 

 and entire leaves, in which one character dominates the other. 



Poultry-breeders have been puzzled for many years by the 

 fact that the Blue Andalusians, a splendid variety of poultry, the 

 breeding of which is with particular success carried on in England, 

 never breed pure. Even in the best strains only about one-half of 

 the young are Blue Andalusians, w T hile the remainder are spotted 

 or black. The most surprising fact, however, is that while the 

 black and the spotted birds prove constant and continue to breed 

 constant, the. blue without exception always produce different- 

 coloured descendants ; if, however, spotted birds are paired with 



