XII.] CONJUGATION AND SEXUAL REPRODUCTION. 167 



and we may maintain that purely parthenogenetic species lose the 

 capability of modifying themselves, more completely, the longer the 

 pure parthenogenesis has continued. So far as we can at present 

 decide, this conclusion is in agreement with facts; inasmuch 

 as no highly developed group of the zoological system, rich 

 in species, is ever entirely composed of purely partheno- 

 genetic species. In the animal kingdom, the Phyllopods and 

 Ostracodes, among the Crustacea, are especially remarkable 

 for the frequency of parthenogenetic reproduction. But pure 

 parthenogenesis only occurs in isolated species, as in the above 

 mentioned Cypris reptans and many other species of the same 

 genus. Among the Phyllopods I only know of one species, 

 Limnadia Hermanni, in which a male has never been found, 

 and it is this very species which seems to have become ex- 

 tremely rare. In the other parthenogenetic species, in addition 

 to the purely parthenogenetic colonies, there are always some 

 which are composed of both sexes, as in Apus cancriformis ; or 

 else a regular alternation of parthenogenetic with bisexual 

 generations takes place in the colony, as in almost all known 

 species of Daphnids. The rich development of these groups of 

 the zoological S3^stem has arisen under the uninterrupted 

 influence of amphigonic reproduction, by means of which 

 variations have been mingled together. It is just the same 

 with the Aphidae (plant-lice and bark-lice), and with the Cyni- 

 pidae. All these groups of animals contain a great variety 

 of species, but, in all, a combination of individual characters 

 takes place from time to time through the fertilization of ova, 

 even though, as is often the case, many purely parthenogenetic 

 generations intervene between the bisexual ones. 



I believe that we find, in the tenacious retention of amphigonic 

 reproduction by such species as the Phylloxera, a strong support 

 of the validity of my theory as to the meaning of sexual repro- 

 duction. Those who still recognize in fertilization a renewal of 

 vital strength, a rejuvenescence, do not require this conception 

 of amphigony as an ever springing well of hereditary individual 

 variation in order to understand its remarkable persistence. 

 But those who agree with me in believing that the partheno- 

 genesis of Cypris reptans which endures for forty consecutive 

 generations is the refutation of any such idea of rejuvenescence, 

 will hardly find another explanation of this tenacious persist- 



