456 LAWS OF MULTIPLICATION. 



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 expenditures for motion. The smaller 

 fertility which habitually accompanies greater bulk, must 

 in all cases be partly ascribed to this. Still, it may be 

 well if we briefly note, for as much as they are worth, 

 the broader contrasts. While a large Mammal bears but 

 a single young one at a time, is several years before it com- 

 mences doing this, and then repeats the reproduction at long 

 intervals; we find, as we descend to the smaller members of 

 the class, a very early commencement of breeding, an increas- 

 ing number at a birth, reaching in small Eodents to 10 or 

 even more, and a much more frequent recurrence of broods: 

 the combined result being a relatively prodigious fertility. 

 If a specific comparison be desired between Mammals that 

 are similar in constitution, in food, in conditions of life, and 

 all other things but size, the Deer-tribe supplies it. "While 

 the large Ked-deer has but one at a birth, the small Eoe-deer 

 has frequently two at a birth.* 



341. The antagonism between growth and sexual gene- 

 * A passage translated for me from the German may be here given in 



