i36 LAWS OF MULTIPLICATION. 



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 

 commences doing this, and then repeats the reproduction at 

 long intervals ; we find, as we descend to the smaller mem- 

 bers of the class, a very early commencement of breeding, an 

 increasing number at a birth, reaching in small Rodents 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 con- 

 ditions of life, and all other things but size, the Deer-tribe 

 supplies it. While the large Red-deer has but one at a 

 birth, the small Roe-deer has two at a birth. 



§ 341. The antagonism between growth and sexual genesis, 

 visible in these general contrasts, may also be traced in the 

 history of each plant and animal. So familiar is the fact 

 that sexual genesis does not occur early in life, and in all 

 organisms which expend much begins only when the limit of 

 size is nearly reached, that we do not sufficiently note its 

 significance. It is a general physiological truth, however, 

 that while the building-up of the individual is going on 

 rapidly, the reproductive organs remain imperfectly developed 

 and inactive ; and that the commencement of reproduction 

 at once indicates a declining rate of growth, and becomes a 

 cause of arresting growth. As was shown in § 78, the ex- 

 ceptions to this rule are found where the limit of growth is 

 indefinite ; either because the organism expends little or 

 nothing in action, or expends in action so moderate an 

 amount that the supply of nutriment is never equilibrated 

 by its expenditure. 



We will pass ever the inferior plants, and limiting our- 

 selves to Phaenogams, will not dwell on the less conspicu- 



