230 THE EVOLUTION THEOEY 



How can the supposed ' formative stimulus ' be so entirely dispensed 

 with in this case ? 



Of course I am well aware that the two kinds of germ-cells 

 have a strong attraction for each other, and that the protoplasm 

 of the ovum actually exhibits tremulous movement when the 

 spermatozoon penetrates through the micropyle, I myself observed 

 this in the case of the lamprey (Petromyzon) when Calberla instituted 

 his investigations on the fertilization of that animal, but has 

 that anything to do with a formative stimulus'? Is it anything 

 more than the result of the chemotactic stimxilus exei-ted by the 

 substance of the ovum upon that of the spermatozoon and conversely ? 

 And have we any ground for seeing anything more in this than an 

 adaptation of the sex-cells to the necessity of mutually finding each 

 other out and thereafter combining 1 Two quite different things are 

 often confused with one another in this connexion : the mutual 

 attraction of the two kinds of sex-cells which tends to secure their 

 union, and the results of this union. A more exact distinction 

 is necessary between the effects and the advantages which allogamy 

 brings in its train and the means by which it is secured in the 

 different species. 



If amphimixis really set up a ' formative ' stimulus, and if the 

 amount of this was regulated by the differences between the two 

 parental germ-plasms, then parthenogenesis, which implies the entire 

 absence of the mingling of two parental cells, would necessarily be even 

 less advantageous than amphimixis between near relatives; but this 

 is not the case. Continued inbreeding leads in many cases to the 

 degeneration of the descendants, and particularly to lessened fertility 

 and even to complete sterility. Thus in my prolonged breeding 

 experiments with white mice, which were later carried on by G. von 

 Guaita, strict inbreeding, effected throughout twenty -nine generations, 

 resulted in a gradually diminishing fertility, and similar observations 

 have been made by Ritzema Bos and others. But why does not the 

 same thing happen in pure parthenogenesis 1 My experiments in 

 breeding parthenogenetic Ostracods (Cyjjris reptans) shows that these 

 crustaceans, in the course of the eighty generations which I have 

 observed till now^ have lost nothing of their prolific fertility and 

 vital power ; and the same is true in free nature of the rose-gall wasp 

 {Rhodites rotOB), which enjoys the greatest fertility notwithstanding 

 its purely parthenogenetic .reproduction, the females not infrequently 



' The cultures were tegun in 1884 and are still continued (March 6, 1902), still 

 multiplying as abunduntly as at the outset, I reckon that there are on an average five 

 generations in a year, which means about eighty generations in sixteen years 



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