An Introduction to a Biology 



albinos at all, and when mated with albinos are dominant 

 over them. I have proved these two kinds to exist. But 

 the existence of the second of them is fatal to the suggested 

 Mendelian theory outlined above. For how could individuals, 

 whose germ-cells appear to bear a new unit g'g, arise from a 

 hybrid in which the germ-cells either carried an element 

 representing g' or g, and never both ? The suggestion that 

 neighbouring ova, or spermatozoa, fuse is not hkely to meet 

 with general approval, yet it is the only one which will 

 account for their appearance if our Mendelian theory is 

 true. We have to choose between the two improbabihties : 

 (i.) that neighbouring germ-cells fuse, and (ii.) that none of 

 the germ-cells of the hybrids bear elements representing 

 animals like them ; and two probabihties : (i.) that neigh- 

 bouring germ-cells do not fuse, and (ii.) that some of the 

 germ-cells of the hybrid contain elements representing animals 

 like them. I choose the probabilities. But, in claiming 

 to have demonstrated the falsity of the Mendelian theory 

 described above, I do not wish to be credited with having 

 " discovered an exception to Mendel's Law " On the con- 

 trary, the best measure of the progress which Mendelian 

 inquiry has made in these two years is the fact that while 

 at the beginning of them the existence of hybrids that breed 

 true would have been regarded as a difficulty, to-day a 

 reasonable explanation of their occurrence has been given. 



Progress in knowledge is made by the suggestion of 

 hypotheses, and their rejection when found to be false ; 

 by this means the MendeHan has been able to account for 

 some very complicated cases of segregation, and for reversion 

 in some cases as well. 



It is natural to inquire how much experiments of this 

 kind tell us about heredity. We are told that Mendel's Law 

 only applies to a very limited class of facts, that there is 

 only a certain very limited and definite set of characters 

 to which it applies, or that it only deals with the phenomena 

 of hybridisation. 



214 



