SECT. 2] AND WEIGHT 497 



and the conclusions of other workers (for which see the section on 

 energy relations), and it is quite certain that the efficiency changes 

 during development. Rubner's law of constant energy requirement 

 during gestation time does not equate well with the findings of 

 Brody, Moulton and others, but it must be remembered that the 

 "law" rests on a very inadequate basis, nothing more, in effect, 

 than a few rough calculations in which some doubtful assumptions 

 are involved. 



Rubner emphasised the fact that the value of about 4000 Calories, 

 which seem to be required to make one kilo of tissue during em- 

 bryonic development, is lower than the corresponding value of about 

 4800 for post-natal life. From this it would appear that the efficiency 

 is greater before birth than afterwards, but this statement needs 

 much qualification. Rubner also showed that the energy consumed 

 per kilo in doubling the birth-weight is approximately the same for 

 different animals. Thus: 



Energy consumption per 

 kilo in doubling 

 Animal the birth-weight, Cals. 



Horse 



Cow 



Sheep 



Pig 



Cat 



Rabbit 



4>5i2 



4^243 

 3>926 



3,754 

 4>304 

 4,554 



Average ... 4j2I5 



Man ... ... ... 28,864 



Robertson has shown that the generalised form of this rule would be 



— = a. log X ■\- h, 



X 



where E is the energy-consumption, x the weight of the animal 

 and a and b constants which are the same for all species except 

 man. 



But we have digressed already too far from the problem of growth. 

 Before taking up the effects of external agents on the growth-rate, 

 it will be worth glancing for a moment at the relations between 

 gestation time and the life-span. Buffon in the eighteenth century 

 and Flourens in the nineteenth maintained that the life-span was 

 five or six times the youth-period (see Lusk), but Weissmann showed 

 that there were too many exceptions to this rule to make it of any 



N EI 32 



