164 



Arend. L. Haeedoorn and A. C. Haeedoorn. 



bringing the heterozygotes together, and once two animals happen 

 to be mated, both pure for the same gene, the family will be pure 

 in respect to this gene. 



Even strict inbreeding, without any selection, as surely must 

 bring about a similar result. In a family of animals, strictlv inbred 

 (brother to sister, parent to clrild), it must necessarily happen, that 

 two are mated, homozygous for the same genetic factor or both 

 lacking it, from which moment the family derived from this single 

 couple, will be pure in respect to that gene. 



Now, did Castle take the only precaution possible, to heighten 

 the probability of starting his selection-experiments with material 

 pure in respect to the genetic factors possibly involved? Did he 

 use material severely inbred for a great number of generations? 



He did not, and this decimates the value of his work. Not only 

 did he not begin his experiments with severely inbred material, but 

 throughout the course of his experiment, the animals were not so bred. 



If Castle's rats were not genetically homogeneous material, he 

 should have seen instances of segregation. Did he see such instances, 

 distribution of any gene over half the number of gametes produced 

 by an animal heterozygous for it ? 



Tlie answer must be: no, he could not see evidences of segre- 

 gation in his pedigrees, because he made no pedigrees of individual 

 rats, but only tables of averages, thoroughly grinding up as it were, 

 and mixing his results before even looking at them. 



The statistical expression of the recorded results of selection- 

 experiments on such a vast scale as those of Castle and his assistants, 

 (they bred over ten thousand rats) is the only reliable way of obscur- 

 ing segregation within Hooded and Irish families, if such segregation 

 occurs. Now, does it occur? Is there any evidence for segregation 

 of genes in Hooded rats, or in Irish rats? 



To find this out, it is obviously useless to work over Castle's 

 results. As useless, as it would be to try and find out the shape of 

 the individual potatoes which went into a dish of "Puree de pommes 

 de terre". If, by chance, there exist any extensive pedigrees of rat- 

 families recorded by pictures, Castle may still do so himself. 



A little more than three years ago, the authors set out to try 

 to find out by carefull pedigree-breeding of Irish and Hooded rats, 

 whether or not there would become visible indications of segregation 

 within these groups. We bought some Hooded rats, and an Irish 



