48 VARIATION. 



presence or otherwise of this gene may make it respond differ- 

 ently to the action of other genes, will even make that in one a 

 case a series of factors, both genetic and non-genetic may in- 

 fluence the further development and in the other case another 

 series. 



If we know from the results of crossing-experiments, that 

 two animals or plants are unlike because of the presence or 

 otherwise of a gene, we need not conclude that the genes them- 

 selves are capable of changing one into the other. In our exam- 

 ple the sugar-corn and the starch-corn, it may be that the thing 

 which finally changes the sugar into starch is present in both 

 kinds, but that the kind of sugar of the two sorts, in the ripe- 

 ning seeds is not of the same nature. It may be that the sugar 

 in sugar-corn can not be converted by the same agent which 

 causes the conversion of the starch-corn's sugar. The difference 

 between the sugars in this case may be due to a presence or ab- 

 sence of any thing, necessary in the longs series of processes 

 leading ultimately to the production of just this kind of sugar 

 which is capable of being converted into starch by a common 

 thing X, which in itself need not be a gene at all. 



If the difference between two individuals, of which one has a 

 gene and the other lacks it is caused by this cooperation or non- 

 cooperation of one single ingredient, one single factor in the 

 series of processes, we see that the same result can be reached 

 in several ways. For the production of pigment in hair or 

 feathers, a whole series of developmental factors must cooperate 

 to the ultimate result. And on several occasions animals may 

 be produced differing from normals in the lack of one gene. 

 The gene need not be the same one, but the result may well be 

 the samerTiamely albinism. 



Instances are known in which albino animals or plants of dif- 

 erent families were mated, and in which the cross produced 

 coloured-offspring, showing that the absence of pigment was in 

 each family due to the lack of a different gene. Baur produced 

 pigmented yoimg from a mating of two white rabbits, Bateson 

 found similar cases both in fowls ^d in the Sweet-pea, and we 



