x translator's preface. 



of the ovum is very differently circumstanced. In its 

 case, the nutriment which is necessary to the con- 

 struction of its organism, cannot be supplied either 

 directly by the yolk or indirectly by a draught upon 

 the mother's supplies ; the egg is too small to admit 

 of the former, and the possibility of the latter is 

 equally precluded by the transient existence of the 

 parent. The materials of growth must therefore be 

 derived from external sources ; the embryo cannot be 

 passive in the undertaking; and hence it is thrown 

 upon the world to provide for its own development. 

 What the membranous uterus and appended blood- 

 vessels supply to the mammal, surrounding objects 

 and the active exercise of its faculties furnish to the 

 insect larva. Every animal, in journeying through 

 the successive phases of development, assumes an 

 immense variety of forms, in fact undergoes meta- 

 morphoses ; in Mammalia, these are concealed from 

 the view of the ordinary observer ; in Insecta they 

 are present to the gaze of the entire world. It is 

 true, then, that all animal forms undergo metamor- 

 phoses, and it only remains to be shown why in some 

 these alterations take place in hidden depths, and in 

 others are exposed to common observation. This 

 brings us to the next great law, which we may thus 

 lay down : — Those creatures whose ova — owing to an 



