346 THE GERM-PLASM 



my expectations : the descendants of the same mother resembled 

 one another as well as the parent with which the experiments 

 began, even as regards minute details of the markings. The 

 differences were mostly as small as those which may be observed 

 in identical human twins : it cannot be stated whether these 

 were due to a want of similarity in the germ-plasm, or whether 

 they were to be accounted for by accidental differences in 

 nutrition. 



Apart from the exceptions about to be mentioned, no modifi- 

 cations occurred even in the course of many generations. I 

 now possess colonies of A and B which cannot be distinguished 

 from their ancestors in the year 1884. Reckoning that from 

 five to six generations were produced each year, about forty 

 generations have been passed through since then. 



In 1887, some individuals of the dark-green variety B appeared 

 for the first time in an aquarium containing the typical yellow- 

 ochre-coloured variety A ; and since then 1 have twice observed 

 a similar occurrence in other broods of A. In the last of these 

 cases, examined in May 1891, it could be proved that only a 

 single Cypris out of 540 adults contained in one aquarium had 

 changed into the dark variety suddenly, and without apparent 

 cause. In another case, intermediate forms between the two 

 varieties were found, as well as ordinary individuals of the 

 variety B ; and this fact is not only of interest theoretically, 

 but it also removes all doubt as to the trustworthiness of the 

 experiments. 



For a long time I waited in vain for the occurrence of the 

 reverse transformation from the dark variety to the light-coloured 

 one, and was inclined to consider that the former was the ances- 

 tral form. But in the winter of 1890-91, a colony of B appeared 

 in which a few typical individuals of the variety A were found, 

 together with typical specimens of B which had been bred in this 

 aquarium for many years. 



It is out of the question that these .sudden transformations 

 were due to external influences, for both forms made their 

 appearance together in the same small aquarium, under precisely 

 the same conditions. We can only suppose that modifications 

 in the composition of the germ-plasm must ha\-e taken place, 

 and I think it is possible to prove that this was the case. 



Parthenogenesis, using this term in the strict sense, has in all 

 cases been derived from sexual reproduction, as is proved in 



