l82 Arend. L. Hagedoorn and A. C. Hagedoorn. 



by analyzing the nature of such exceptions, by experimenting, by 

 analyzing the conditions, find a "law", which lies at the base of the 

 rule. To a "law" there are no exceptions. If there is one instance 

 found, which proves that the "law" is not general, it ceases to be a 

 law, and again we have to try and uncover our real law. 



It may happen, and in fact, it happens not unfrequently, that 

 to all appearances a rule and a law are flatly opposed. To give an 

 example. We all know that heavy bodies fall with greater velocity 

 than lighter ones. This is a rule. It is only in very exceptional 

 circumstances that bodies of different specific gravity fall with the 

 same velocity, namely, if the room in which they fall is void of air. 

 Still, because by such experiments we know that it is the resistance 

 of the air which causes lighter bodies to faU slower, Newton's law 

 is not invalidated by our every-day experience. 



Neither of the so-called laws of Mendel may be called a law. 

 Neither the rule of Dominance, nor the rule of independant segre- 

 gation. But, through the experiments which show occasional cases 

 of coupling or of mutual repulsion of genes, we know at last that 

 for every gerie, which entered only in one gamete of an organism's 

 germ, as many gametes are always produced with as without this 

 gene. This again beautifully illustrates the difference between a rule 

 and a natural law. The rule is that on mating inter se two animals 

 (on selffertilizing a plant), heterozygous for one gene, we get an off- 

 spring, consisting of three times as many individuals with as such 

 without this gene. To this rule there may be found exceptions. For 

 instance, the normal hybrids from waltzing and normal mice, when 

 mated inter se, always produce much less than 25% of waltzing 

 young. 



But to this law there are no exceptions: "If a certain gene enters 

 in only one of the two gametes, from which an organism grows up, 

 this organism will in its turn produce as many gametes with this 

 gene, as such without it." 



In regard to selection, we again have a rule and a law, which 

 on superficial inspection seem flatly to contradict each other. The 

 rule is that "by selection on any character, the mean of the group 

 subjected to this selection, in respect to this character, is shifted into 

 the direction of the variates selected". By a study of the exceptions 

 to this rule (inefficiency of selection in 'pure lines and in clones) at 

 last this law may be evolved: "Selection can not influence a gene", 

 to which law there are no exceptions. 



