148 LAWS OF MULTIPLICATION. 



is part of the material from which a young one is built up, 

 it is part of the outlay for reproductive purposes, and diminu- 

 tion of it is a loss of reproductive power. Indeed the case 

 aptly illustrates, under another aspect, the struggle between 

 self-preservation and race-preservation. Maintenance of the 

 cow's life depends on maintenance of its heat ; and main- 

 tenance of its heat may entail such reduction in the supply 

 of milk as to cause the death of the calf. 



Evidence derived from the habits of the same or allied 

 genera in different climates, may naturally be looked for ; but 

 it is difficult to get, and it can scarcely be expected that the 

 remaining conditions of existence will be so far similar as to 

 allow of a fair comparison being made. The only illustrative 

 facts I have met with which seem noteworthy, are some named 

 by Mr. Gould in his work on The Birds of Australia. He 

 sa}^s : — " I must not omit to mention, too, the extraordinary 

 fecundity which prevails in Australia, many of its smaller 

 birds breeding three or four times in a season ; but laying 

 fewer eggs in the early spring when insect life is less 

 developed, and a greater number later in the season, when 

 the supply of insect food has become more abundant. I have 

 also some reason to believe that the young of many species 

 breed during the first season, for among others, I frequently 

 found one section of the Honey-eaters (the Melithrepti) 

 sitting upon eggs while still clothed in the brown dress of 

 immaturity ; and we know that such is the case with the 

 introduced Gallinacece (or poultry) three or four generations 

 of which have been often produced in the course of a year. " 

 Though here Mr. Gould refers only to variation in the 

 quantity of food as a cause of variation in the rate of 

 multiplication, may we not suspect that the warmth is a 

 part- cause of the high rate which he describes as general? 



§ 349. Of the inverse variation between activity and 

 genesis, we get clear proof. Let us begin with that which 

 Birds furnish. 



