182 THE THEORIES OF EVOLUTION 



shells and banded shells being in the exact proportion 

 of three to one. 2 



Correns crossed two varieties of nettles, Urtica 

 pilulifera and Urtica dodartii, which differ only in the 

 appearance of the leaf's blade, dentate in one case, 

 entire in the other. Results from the cross confirmed 

 Mendel's forecast. 



Besides these simple cases, scientists have experi- 

 mented with correlated characters, characters reacting 

 upon one another, types presenting two dominant and 

 two recessive characters allied, crosses between pure 

 dominants and impure dominants, etc. Leaving 

 aside such special and complicated cases, we will en- 

 deavour to estimate the theoretical importance of 

 Mendel's laws. 



The existence of independent characters which 

 never blend with one another and may vary inde- 

 pendently, the existence of character-units, is the con- 

 clusion Mendel himself drew from his experiments. 

 The germ cells contain, he thinks, concrete represent- 

 atives of those characters which, at the time of cross 

 fertilisation, combine in such a way that the represent- 

 atives of one character only become active. The pres- 

 ence of representatives of the other characters in the 

 hybrids' germ cells reveals itself only in the offspring. 



2 A. Lang. Ueber die Mendelschen Gesetze, Art und Varietatenbild- 

 ung, Mutation und Variation, insbesondere bei unsem Hain-und Garten- 

 schnecken. (Verh. schweiz. JYaturf. Ges. 1905, Luzern.) 



