D. J. SCOURFIELD ON MENDELISM AND MICROSCOPY. 415 



characters suitable for observation a rather troublesome matter. 

 Again, the conditions necessary for the healthy development of 

 microscopic animals in captivity is not a subject about which very 

 much is known, although there .seems no reason why the com- 

 moner and hardier species should not be reared in such quantities 

 and as long as desired. But the main difficulty would be that of 

 discovering the forms which would give fertile hybrids, for there 

 are here no records to serve as a guide such as exist for domesti- 

 cated animals and plants. It is not even known for certain 

 whether hybrids can be obtained, let alone fertile hybrids. 

 Attempts have, to be sure, been made to get hybrids between 

 different species of microscopic animals, but without positive 

 result, so far as I know. Some years ago I made an unsuccessful 

 effort to obtain a cross between Cyclops serrulatus and C, 

 prasinus, and Richard, I believe, tried the same thing with 

 C.fuscus and C. albidus. Hacker also has more recently experi- 

 mented with Diaptomus gracilis and D. denticoniis, but again 

 without success. Nevertheless, there are no grounds for 

 thinking that crosses are impossible. On the contrary, it is 

 suspected that, even under natural conditions, crosses may some- 

 times occur. For example, a peculiar form of Cyclops (C. 

 bistriatus, Koch = C. distinctus of Richard, Schmeil, etc.), which 

 is evidently very closely related to C. fuscus and C. albidus, 

 certainly looks as if it might be a hybrid between these two 

 species. It possesses some characters of C. fuscus, such as the 

 fine setae on the inner sides of the tail-fork, and to some extent 

 also the dark coloration, while it resembles C. albidus in having 

 only a plain knife-like ridge on the last joint of the first antennae 

 and in having divergent ovisacs. The latter, although not 

 quite so divergent as in C. albidus, are very different from the 

 adpressed ovisacs of C. fuscus. Whether it is a hybrid or not has 

 never been definitely determined, and it would be a useful piece 

 of work to follow the development of its progeny, as would also 

 the parallel endeavour to procure an artificial cross between 

 C. fuscus and C. albidus. 



But the want of success hitherto experienced in obtaining 

 hybrids between microscopic animals may very well be due to the 

 fact that the forms used have been too far apart. It would 

 undoubtedly be better to start with quite closely allied varieties. 

 Among the fresh-water Entomostraca, the group with which I 



