go PROBLEMS OF RELATIVE GROWTH 



Thus it would appear that one of the chief advances made 

 by man in creating improved breeds of sheep and other meat 

 animals has been simply to steepen growth-gradients which 

 already operate during post-natal development in the wild 

 ancestral forms. Hammond himself (1927) has expressed a 

 similar idea. ' The improver of meat-producing animals has 

 apparently not chosen mutations occurring in isolated points 

 independently, but rather has based his selection on the 

 generalized correlated changes of growth '. (See also Fig. 96, 

 p. 223.) 



In consequence of the gradient, there will be much less 

 difference in the size of the metatarsal between a semi-wild 

 and an improved breed than in the size of the femur, This 

 is well brought out by Hammond (1927) in his Fig. 4. 



In this connexion, it is well to remember that during em- 

 bryonic life, the limbs of sheep must show a growth-gradient 

 precisely opposite in sign to that of their post-natal period. 

 Lambs are born with relatively long legs, as an adaptation 

 to accompanying their dams almost from birth. To achieve 

 these unusual proportions, the leg must have exhibited posi- 

 tive heterogony during foetal life ; and to allow for the fact 

 of the later centre of negative heterogony in the cannon-bone, 

 this same region must have been the positive growth-centre 

 in the earlier period. The same reasoning applies to Ocypoda, 

 whose young are similarly precocial. 



§ 4. The Form of Growth-gradients 



Analysis of the data of Kemp and his co-workers on Palae- 

 mon spp. undertaken by Miss I. Dean (unpublished) gives 

 a further interesting result. In these prawns, both male and 

 female have obviously heterogonic chelae, but the male's 

 heterogony is considerably higher. Thus a male and a female 

 of the same absolute size will possess chelae of very different 

 sizes, the female's being considerably the smaller. But if we 

 take a male chela and a female chela of the same absolute 

 size (which will of course be borne by a small male and a 

 large female body) the proportions of the separate joints will 

 be found to be fairly similar. This indicates that whenever 

 marked heterogony, or at any rate heterogony designed to 

 give rise to a large chela, is present, it must operate by essen- 

 tially the same growth-mechanism within the limb (and a 

 mechanism quite different from that in a slightly heterogonic 

 pereiopod), whether the growth-coefficient of the whole limb 



