GENERATION OF ANIMALS, IV. iv. 



animals which produce many offspring the fetations 

 are many in number and a single continuous one does 

 not result instead of many — viz., a fetation is not 

 formed out of any casual quantity : if there is too 

 little or too much, none will be formed, because there 

 is a definite limit set both to the dynamis of the 

 material which is acted upon and to that of the heat 

 which acts upon it. Similarly also in the case of those 

 animals which are large and produce one offspring 

 only, a large amount of residue does not give rise to 

 a large number of offspring, for the same holds good : 

 here too, the amount of the material and of that 

 which works upon it are definite. So then they do 

 not emit a larger amount of such material, owing to 

 the cause already mentioned; and the material which 

 they do emit is, in the natural course, just sufficient 

 in amount to provide for a single fetation only. If 

 ever more of it is supphed, then twins are produced. 

 And hence, also, such creatures seem rather to be 

 monstrosities, because their formation is contrary to 

 the general rule and to what is usual. Man, how- 

 ever, has a footing in all the classes, producing one 

 offspring, or on occasion, many, or few, though most 

 naturally and normally one is the number : the 

 production of many offspring is due to fluidity of the 

 body and to heat, [since the nature of semen is fluid 

 and hot ;] of few or of one, to the size of the body. 

 And to this it is due also that in man alone among 

 the animals is the period of gestation of variable 

 length " : other animals have a single period, but 

 with man there are several : children are born at 

 seven months and ten months and at intermediate 

 times, and indeed eight months' babies live, though 

 less often than the others. The reason may be 



439 



