GROWTH AND SEXUAL GENESIS. 435 



among domestic creatures that are similarly conditioned, 

 and closely -allied by constitution, may be held to show, 

 more clearly than most other contrasts, the inverse varia- 

 tion between bulk and sexual genesis ; since here the 

 cost of activity is diminished to a comparatively small 

 amount. There is little expenditure in flight — sometimes 

 almost none ; and the expenditure in walking about is 

 not great : there is more of standing than of actual 

 movement. It is true that young Turkeys commence 

 their existences as larger masses than chickens ; but it is 

 tolerably manifest that the total weight of the eggs produced 

 by a Turkey during each season, bears a less ratio to the 

 Turkey's weight, than the total weight of the eggs which a 

 Hen produces during each season, bears to the Hen's weight ; 

 and this is the fairest way of making the comparison. The 

 comparison so made shows a greater difference than appears 

 likely to be due to the different costs of locomotion ; con- 

 sidering the inertness of the creatures. Remembering that 

 the assimilating surface increases only as the squares of the 

 dimensions, while the mass of the fabric to be built up by the 

 absorbed nutriment increases as the cubes of the dimensions, 

 it will be seen that the expense of growth becomes relatively 

 greater with each increment of size ; and that hence, of two 

 similar creatures commencing life with different sizes, the 

 larger one in reaching its superior adult bulk, will do this at 

 a more than proportionate expense ; and so will either be 

 delayed in commencing its reproduction, or will have a 

 diminished reserve for reproduction, or both. Other orders 

 of Birds, active in their habits, show more markedly the con- 

 nexion between augmenting mass and declining fertility. 

 But in them the increasing cost of locomotion becomes an 

 important, and probably the most important, factor. The 

 evidence they furnish will therefore come better under 

 another head. Contrasts among Mammals, like 



those which Birds present, have their meanings obscured by 

 inequalities of the expenditure for motion. The smaller 



