202 



PROBLEMS OF RELATIVE GROWTH 



logical cause), ample nutrition at the period of maximum 

 growth, followed or preceded by undernutrition at other 

 periods, will produce an animal with the organ of unusually- 

 large relative size ; e.g., in rabbits, the ears have an early 

 period of maximum growth : accordingly maximal feeding 

 when young, when followed by low feeding later, will produce 

 a rabbit with ears above the normal proportionate size. 

 Hammond further points out that since in meat animals the 



maximum percentage of 

 flesh is only produced by 

 a high level of feeding 

 throughout most of the 

 growth-period, animals 

 cannot show their full 

 potentialities as meat- 

 producers if kept in un- 

 favourable nutritive con- 

 ditions. When practising 

 genetic selection for meat 

 qualities, it is therefore 

 necessary to provide 

 optimum conditions for 

 individual development 

 in order to be able to 

 pick out extreme vari- 

 ants. Conversely, if a 

 breed is required for a 

 poor environment, selec- 

 tion should be practised 

 with sub-optimal nutri- 

 tion, of the type to be 

 expected in this environ- 

 ment. 



In regard to plants, 

 numerous examples are 

 known of external conditions affecting proportions. We may 

 cite two papers of Pearsall and Hanby (1925 and 1926). In the 

 former they showed that in Potamogeton leaves excess calcium 

 causes a predominance of growth in breadth, excess potassium a 

 predominance of growth in length. This was due partly to 

 the greater relative length of the individual cells in the Ca 

 cultures, partly to the arrangement of cells in the young 

 leaf-initials. In the latter paper they found that reduced 



Controls at 3 weeks Constant > 10 weeks Controls at 10 weeks 



Fig. 89. — Diagram showing changes pro- 

 duced in relative weight of various parts 

 of the body in young rats held at constant 

 weight for 7 weeks by under- feeding (centre) 

 as compared with controls at the beginning 

 (left) and end (right) of the experiment. 



