TRANSLATOR S PREFACE. IX 



The expression itself, simply meaning the develop- 

 ment of generations, does not involve a theory, 

 although it is associated with one. It simply implies 

 a fact patent to all zoologists. The theory, which is 

 actually the embodiment of the law sought for, ex- 

 presses itself in the following words : — The formation 

 of new individuals may take place, in some instances, 

 by gemmation from, or division of, the parent-being ; but 

 iMs process is an exhaustive one, and cannot be carried 

 on indefinitely ; when, therefore, it is necessary to 

 insure the continuance of the species, the sexes must 

 present themselves, and germ and sperm must be 

 allowed to come in contact ivith each other. 



There is another philosophic generalization framed 

 by the author, and which it appears to us should 

 follow, rather than precede the first. It relates to 

 the manner in which the animal proceeds from what 

 it has been as an ovum, to what it is in its adult con- 

 dition. If we watch the development of any mammal, 

 we shall see that during its entire progress from 

 simple to complex, from an embryonic to a mature 

 state, it has been confined not only within the uterus 

 of the mother, but also within those membranous 

 folds which really constitute its egg-shell. But, on 

 the other hand, if we turn our attention from the 

 mammal to the insect, we shall find that the product 



