204 EXPERIMENTAL EVOLUTION LECT. 



than is usually the case because it lengthens in pur- 

 suit of richer soil ; and when thick-rooted plants, such 

 as beet, are experimented upon, a very striking differ- 

 ence is produced in the length and also the thickness 

 of the root by the chemical constitution of the soil 

 the plants inhabit. 



Many facts of this sort might be quoted. Con- 

 cerning animals, we may note that Bourguignat thinks 

 left-handed shells of molluscs are due to some electrical 

 influences in operation during the period when the 

 embryo is rotating in the egg (experiment might 

 settle this) ; and Brot 1 has noticed a fact which, 

 although very singular, can be merely mentioned 

 here. One year he remarked a small pond near Geneva 

 which contained many pond-snails, and found that 

 nine-tenths of them were in various respects abnormal. 

 Next year he visited the place again, but found only 

 normal forms. The fact remains unexplained but 

 not unparalleled, as other observers have met with 

 similar instances as our writer only notices that in 

 the first visit he found a large amount of common 

 Hydra, while none at all were to be found at the 

 second. It would seem rather audacious to ascribe 

 the deformations of the molluscs to the presence of 

 Hydra, even in large quantities ; but, on the other 



1 Cf. Locard, Variations Malacologiques, vol. ii., where these facts 

 are noted. 



