256 



Eeseakch in Animal Breeding. [June, 



s. character as that of weight, where inheritance is seemingly of 

 a blended nature, can nevertheless be interpreted in terms of 

 ■definite factors, each producing a definite effect. It is not of 

 course suggested that weight is dependent solely upon such 

 factors.- Absolute uniformity, even where animals are of the 

 same genetical constitution, cannot be expected. For no two 

 animals can be treated exactly ahke with respect to food and 

 other conditions. Moreover, it is conceivable that other factors, 

 influencing vigour as distinct from weight, may come into opera- 

 tion, and produce some effect upon weight itself. 



The results are not without interest in connection with the 

 problems of in-breeding and the effects of a cross. Close in- 

 breeding is held by some to lead to deterioration in the matter 

 of size, and there is certainly some foundation for this belief. 

 Yet it is by no means certain that, sometimes at any rate, this 

 deterioration is not due to the fact that the original material was 

 impure in some of the size factors, and that one or more of these 

 may have been eliminated by unconscious selection. Again, 

 there is much evidence to suggest the vievr that first-cross animals 

 frequently make unusually good growth, and exceed both parental 

 strains in weight. By some this effect is referred to the increased 

 vigour resulting from a cross. This, of course, is no explanation, 

 so long as we cannot state precisely how this increased vigour is 

 l)rought about. It may be that there are definite factors working 

 for vigour, though at present this has not been experimentally 

 proved. The poultry results force us to recognise that increased 

 size in first crosses may be due to a cumulative effect of different 

 size factors brought in by the two parental breeds. 



The two strains AABBccdd and aahhCCDD would each be of 

 intermediate size, and nearer in this respect to the Sebright than 

 to the Hamburgh. First-cross birds betew^en these two would 

 be in constitution AaBhCcDd, i.e , of the same constitution as 

 the Fl Hamburgh-Sebrights. They would be larger than either 

 of the intermediate parental strains, but this increase would not 

 be due to vigour incidental to a cross, but to the cumulative effect 

 of the 4 factors A, B, C, D, of which two were brought into the 

 cross by each parent. Moreover, such Fl birds might be 

 expected to give a small proportion of progeny larger than them- 

 selves, and breeding true to this increase in size. Where a notable 

 increase in size follows on a cross, it suggests that the breeds 

 used contained different size factors ; and if this w^ere so it would 

 be possible to establish a strain of increased size by working on 

 the lines indicated by the factorial theory. 



