GENERATION OF ANIMALS, II. vi. 



the uterus ; e.^., things which we boil to make them 

 pleasant for food, or for any other practical purpose. 

 The only difference is that in this case the correct 

 proportion of heat" to suit the movement ** is supplied 

 by us, whereas in the other, it is supplied by the 

 nature of the generating parent. With those animals 

 that are formed spontaneously the cause responsible 

 is the movement and heat o^the climatic conditions. 

 Heat and cooling (which is deprivation of heat) are 

 both employed by Nature. Each has the faculty, 

 grounded in necessity, of making one thing into this 

 and another thing into that ; but in the case of the 

 forming of the embryo it is for a purpose that their 

 power of heating and cooHng is exerted and that 

 each of the parts is formed, flesh being made soft — as Flesh, 

 heating and cooling make it such, partly owing to 

 necessity, partly ybr a purpose, — sinew soUd and elastic, 

 bone solid and brittle. Skin is formed as the flesh skin. 

 solidifies, just as scum or " mother " forms on boiled 

 hquids. Its formation is due not merely to its being 

 on the outside, but also to the fact that glutinous 

 substance remains on the surface because it cannot 

 evaporate. In blooded animals the glutinous sub- 

 stance is more fatty than in bloodless ones, in which 

 it is dry, and on this account the outer parts of the 

 latter are testaceous or crustaceous. In those blooded 

 animals whose nature is not excessively earthy, the 

 fat collects under the protective covering, the skin, 

 which seems to indicate that the skin is formed out 

 of this sort of glutinous substance, since of course 

 grease is to some extent glutinous. We are to say, 

 then, as already stated, that all these things are 

 formed partly as a result of necessity, partly also not 

 of necessity but ybr a purpose. '^ 



223 



