1918. J N. Annandale : Molluscs of the InU Lake. 175 



these theories have been put forward with the same wealth of natural 

 illustration that Darwin gathered together in his Origin of Specie-'^, 

 and it is just as important that observations should be continued in the 

 field on as large a scale as possible, and without reference to any one 

 preconceived theory, as that experiments should be conducted in the 

 laboratory or garden-plot with a theory to support. I do not think 

 that any single formula can express, much less explain, evolution. 



I am of the opinion that the Inle shells illustrate two different and 

 possibly somewhat exceptional lines along which evolution may proceed. 



Fig. 9. — Genitalia of Limnaea mimetica. 



Ac. g. — accessory gland. Al. g. — albumen gland. H. G. — hermaphrodite gland. 

 •P. — prostate. P. S. — penis-sheath. Sp. — spermatheca. U. — uterus. 



We know that in some forms of Limnaea the plasticity is of the 

 young individual, and that modification can be produced and re- 

 produced in either direction from one generation to another. Probably, 

 however, even in comparatively simple shells such as those of Limnaea, 

 a time wovdd come, if the environment were constantly changed in 

 one direction, at which plasticity disappeared. This is indicated by 

 Whitfield's observation that individuals of this genus if kept in 

 captivity for several generations lost their monoecious structure and 

 became dioecious. He was of the opinion, that this was directly due 

 to abortion of that part of the shell in which certain of the sexual 

 organs were normally lodged. It follows, therefore, that in suggesting 

 that altered environment may finally result in a modification of the 

 race rather than of individuals, one is not necessarily expressing 

 heretical views as to the inheritance of acquired characters — a series 

 of phenomena, or supposed phenomena, which it seems to be quite 

 impossible either to prove or definitely to disprove. If the organs of 

 one sex in a monoecious animal can disappear after several generations 

 as a direct result of a change of conditions on the shape of inanni-, 



