TRANSLATOR S PREFACE. Vll 



he .has brought together all the facts bearing upon 

 the subject of generation, and by a cautious and 

 impartial comparison of them, has succeeded in 

 detecting that hitherto, to us, obscure scheme which 

 governs the production of animal forms. 



The immortal Harvey gave rise to the aphorism 

 that everything living proceeds from an ovum; but 

 though in his time, as in the present day, his dictum 

 could, not be disproved, there was a period in the 

 annals of natural science when it appeared to be little 

 better than a pure assumption. The reader is doubt- 

 less aware that, among the higher animals, the forma- 

 tion of new individuals takes place invariably in one 

 and the same manner ; — by the contact of sperm and 

 germ, an ovum is developed, which, in its turn, gives 

 rise to a being similar to those from which it pro- 

 ceeded. The discoveries of naturalists, however, 

 demonstrated apparently that the production of indi- 

 viduals does not depend upon this combination of two 

 dissimilar factors. This circumstance inaugurated an 

 entirely new era in the history of natural science. It 

 was found that certain insects {Aphides), entirely devoid 

 of both germ- and sperm-producing organs, possess 

 the power of bringing into the world new beings like 

 themselves ; males and females being absent, and the 

 animals which originate the offspring being essen- 



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