II 



ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 33 



of the nest. The methods of three species, Ghelifer cyrneus, < '. latreillii, 

 and Obisium muscorum, were observed in detail. They were essentially 

 identical and probably characteristic of all Pseudoscorpiones. 



«. Crustacea. 



Inheritance in Parthenogenesis.* — W. E. Agar has made important 

 observations on inheritance in three Cladocera, Simocnphalus exspinosus, 

 S. vetnlus, Daphnia obtusa, and Macr o siphon anther inii&n Aphid. Where 

 it is necessary to express the group of individuals descended asexually (in 

 Agar's cases parthenogenetically) from a single ancestor. Small's word 

 clone " is used. When the population is known to have been descended 

 asexually from a single common ancestor, it consists of a single clone and 

 may be called monclonal. When it is composed of a number of clones 

 each descended from an original ancestor not asexually connected with 

 the original ancestors of the other clones, the population may be called 

 polyclonal. 



The general result of the experiments with Cladocera was to show a 

 genetic identity between parent and offspring. In the experiment with 

 the Aphid there was a hint of a partial inheritance of individual varia- 

 tions, but it was found that this resemblance was probably not due to 

 inheritance proper. No mutation was observed in the Cladocera. Con- 

 spicuous variations when they occurred were tested by breeding, and 

 found to show no trace of inheritance. Such variations must therefore 

 be regarded as purely somatic modifications. Purely somatic fluctuation 

 is increased by subjection to abnormal conditions. 



Parthenogenetic and Sexual Reproduction in Cladocera.* — W. E. 

 Agar publishes a paper which presents some new experimental evidence, 

 and points out some general indications that the change from partheno- 

 genetic to sexual reproduction, and the degeneration which so often ac- 

 companies it, is determined by environment only, and that the number of 

 preceding parthenogenetic generations, or the lapse of time since the last 

 sexual act are not, as such, relevant to the matter at all. His conclusions 

 refer mainly to Simocephalus vetulus, but may probably be safely extended 

 to a wide range of the Cladocera. They are as follows : — Certain not 

 yet fully elucidated factors in the environment influence the onset of 

 sexuality. Certain factors likewise bring about " degeneration," or high 

 rate of mortality. Certain factors of the environment may act cumu- 

 latively over a number of generations. Therefore the increasing sexuality 

 and " degeneration " (or high mortality) observed under certain sup- 

 posedly constant experimental conditions receive a ready explanation in 

 the supposition that the environment is one favourable to the develop- 

 ment of these phenomena. This explanation is made much more 

 probable when we find that under other experimental conditions there is 

 no tendency to increasing sexuality or degeneration. Many species 

 exhibit the phenomenon of specially labile periods, when sexuality is 



* Phil. Trans., Series B, ccv. (1914) pp. 421-89. 

 t Journ. Genetics, iii. (1914) pp. 178-94. 



Feb. nil,, 1915 I- 



