14 Genetic Studies in Rabbits 



hardly warrant us in attaching much significance to this feature. 

 Greater interest attaches to the fact that the maturity age here is less 

 dependent upon weight than in the case of the Flemish-Polish cross. 

 It is true that the group of lowest average weight (6 lbs. 3 oz.) matured 

 early and that the heaviest group (7 lbs. 12 oz.) matured late, but apart 

 from these groups there is no tendency in earlier maturity being asso- 

 ciated with lower weight. An association of increased weight with later 

 maturity is rather more marked in the case of the F.^ generation, but 

 the fewer numbers together with the smaller range of variation both 

 detract from its significance. 



The possibility that weight might be affected by such features as 

 size of litter, age of doe, and season of birth naturally occurred to us, 

 but examination of our records from this point of view has not led to 

 any positive result. In this respect we agree with MacDowell ('14, 

 pp. 27-28) and we do not think it necessary to go more fully into an 

 analysis of our data in this connection. Castle ('09, p. 39) considers that 

 variation in size is to some extent dependent upon variation in the 

 quality of food. Davies ('17) on the other hand holds that, provided the 

 food supply is adequate, size is a matter of heredity. Our general 

 impression is that the last-named author is right, but we recognise that 

 the point can only be definitely settled by definite experiment. We have 

 endeavoured to keep our animals as far as possible under uniform con- 

 ditions throughout the year. The hutches are all in a large shed free 

 from draughts and warmed in very cold weather. Until quite recently 

 the animals have always had a liberal allowance of oats together with 

 such green food, roots, etc. as the season affords. They are bedded on 

 sawdust and hay and we have hitherto been very free from sickness — a 

 feature which is at once brought out by the growth curves (c£ McDowell, 

 '14, p. 43). 



On the whole our experience suggests the following tentative con- 

 clusions with regard to rate of growth and maturity. 



(1) The growth curve normally shews a steady rise, gradually 

 flattening with age. About the period of sexual maturity it usually 

 receives a definite check. As a rule this is temporary and the animal 

 subsequently becomes somewhat heavier than it was at maturity^ 



(2) Though animals belonging to large breeds may mature more 

 slowly than those belonging to small breeds it does not follow that age 



1 In the rat growth is apparently continuous, and there would appear to be no definite 

 check at sexual maturity (cf. Donaldson, '15, pp. 66 and 67). 



