468 LAWS OF MULTIPLICATION. 



maintenance of heat diminishes the rapidity of propagation, 

 is difficult to obtain. It is, indeed, obvious that the warm- 

 blooded Vertebrata are less prolific than the cold-blooded; 

 but then they are at the same time more vivacious. Simi- 

 larly, between Mammals and Birds (which are the warmer- 

 blooded of the two) there is, other things equal, a parallel, 

 though much smaller, difference; but here, too, the unlike- 

 nesses of muscular action complicate the evidence. Again, 

 the annual return of generative activity has an average cor- 

 respondence with the annual return of a warmer season, 

 which, did it stand alone, might be taken as evidence that a 

 diminished cost of heat-maintenance leads to such a surplus 

 as makes reproduction possible. But then, this periodic rise 

 of temperature is habitually accompanied by an increase in 

 the quantity of food a factor of equal or greater importance. 

 We must be content, therefore, with such few special facts as 

 admit of being disentangled. 



Certain of these we are introduced to by the general rela- 

 tion last named the habitual recurrence of genesis with the 

 recurrence of spring. For in some cases a domesticated crea- 

 ture has its supplies of food almost equalized ; and hence the 

 effect of varying nutrition may be in great part eliminated 

 from the comparison. The common Fowl yields an illustra- 

 tion. It is fed through the cold months, but nevertheless, in 

 mid-winter, it either wholly leaves off laying or lays very 

 sparingly. And then we have the further evidence that if it 

 lays sparingly, it does so only on condition that the heat, as 

 well as the food, is artificially maintained. Hens lay in cold 

 weather only when they are kept warm. To which fact may 

 be added the kindred one that "when pigeons receive arti- 

 ficial heat, they not only continue to hatch longer in autumn, 

 but will recommence in spring sooner than they would other- 

 wise do." An analogous piece of evidence is that, in 

 winter, inadequately sheltered Cows either cease to give milk 

 or give it in diminished quantity. For though giving milk 

 is not the same thing as bearing a young one, yet, as milk 



