Ill', D. J. SCOURFIELD OX MENDELISM AXD MICROSCOPY. 



am most familiar, and the one which seems to me to be specially 

 marked out for experimental work on the lines suggested, many 

 such closely related forms occur. For example, the striking 

 black and white form of Diaptomus laticeps already mentioned, 

 although so different in appearance, is probably only a sub- 

 variety of the species, and as such would most likely produce 

 fertile progeny if crossed with forms from other Scottish lochs. 

 In this case there would be at least three characters to watch 

 — namely, the blue colour, the brown colour, and the distri- 

 bution of the colours. In Cyclops jimbriatus and its variety 

 poppei we have a pair of forms which only differ, so far as is 

 known, in what is probably a single character — the shape and 

 spinous armature of the furcal lobes. Unfortunately, in this 

 [(articular case, the variety is very rare. Somewhat similar 

 cases of forms practically differing only in single characters are, 

 however, also to be found in commoner species, as among the 

 varieties, sub-species, species, or whatever they may be, grouped 

 under the names of Cyclops strenuus and Cyclop)s serrulatus. 

 Again, among the Daphnids there are several very closely related 

 forms, which are nevertheless not merely due to local or seasonal 

 influences. I think the common Daphnia pulex and D. obtusa are 

 such a pair, also Cerlodaphnia reticulata and its variety serrata, 

 and Cerlodaphnia quadrangula and its variety hamata. With 

 these Cladocerans, of course, a complication arises owing to the 

 normal occurrence of many generations of parthenogenetic 

 females interpolated between the sexual generations ; but, as 

 already pointed- out, this would really add to the interest of the 

 experiments, and, in order to save time, it is not impossible that the 

 number of the asexual generations might be reduced by artificial 

 means. A low temperature would probably do this, for it is known 

 from the researches of Ekman that in the arctic regions the asexual 

 generations may not be more than one or two, even in species which 

 in temperate regions normally produce very many more. 



In the above suggestions regarding the utilisation of micro- 

 scopic animals for experimental work on heredity I have only 

 referred to the Entomostraca, but it is quite possible that other 

 groups could also be pressed into service. The Aphides, for 

 example, would probably furnish material for such work (they 

 have already been used for some important studies on the germ- 



11s, the results of which seem to distinctly favour the idea of 



