258 Castle. 



tbat is those free from variation in the hooded coat-pattern, notwith- 

 standing the fact that selection lias been made uniformly in one direction 

 for 15 generations. 



2. My critics maintain that T should have inbred my stock. 

 Since the entire stock is descended from a very few individuals (less 

 than a dozen) and we have at no time hesitated to mate together brother 

 and sister, provided they varied in the same direction, but have always 

 used the most extreme individuals (plus or minus) which were available, 

 to mate with each othei-, it follows that very close inbreeding must have 

 occurred throughout the experiment. What ground my critics had for 

 assuming that inbreeding had not been practised is not cleai-. 



3. The serious charge is made (page 16-t) and emphasized by 

 italics that I "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". This statement is untrue. The pai-entage 

 of each rat is recorded as its descriidion is entered in the record book. 

 Doubtless my critics meant to say no pedigrees had been published, 

 which is a very different matter, luit one which presents some difficulty 

 when one is dealing with 2i),(i(i(i individuals. My critics' use of language 

 is here inexcusably careless. 



4. 1 am further charged (page 17(i) and again in italics, with 

 failure to "select rigorously". What 1 said concerning this matter 

 in the paper on which the criticism is based was this, "In each gen- 

 eration the most extreme individuals were selected as parents". This 

 statement apparently has no weight with my critics. J also mentioned 

 the exti-eme minus variants at the outset of the experiment to have 

 been of grade — 2 (witlnuit back-stripe) and am in consequence criti- 

 cised for using animals of any other grade in tiie minus series. Had 

 1 folldwed this advice, iny exi)eriments would have ended as abruptly 

 as those of aiy critics, "in a catastrophe", for there was an insufficiency 

 of such animals at the outset, and such extreme variates rarely produced 

 their like. In the first two generations of offspiing from minus selected 

 parents there were only 7 in 1K7 individuals as extreme as — 2 in 

 grade and im two id these (if iipiiosite sex were available at the same 

 time bii mating with each dther. In the next generation there were 

 16 in 195 young which were as extreme as this in grade, and for the 

 fii'st time a pair of these became available as parents. They produced 

 9 young, of which only two Avere as extrenu^ in (diaracter as their 

 parents. My critics seem to have adopted the idea that the minus 

 variation must tie due to "loss of genes" and so it is inconceivable to 



