206 EXPERIMENTAL EVOLUTION LECT. 



many years before, were made to run into a garden. 

 The water was ferruginous, and its temperature was 

 32 or 33 Cent, in 1881, when the writer first began 

 to notice the facts. At that time individuals of 

 Physa acuta were seen in the water, and it was re- 

 marked that they were generally small, and in many 

 cases much deformed, as one may perceive by a 

 glance at the illustrations which accompany M. 

 Regelsperger's paper. In 1882 the temperature of the 

 water had fallen off considerably ; instead of being at 

 32 or 33 it had decreased to 26*5. M. Regelsperger 

 again examined the animals, and saw that deformed 

 individuals were very scarce. Since then the flow of the 

 water has entirely ceased, the pipes having become im- 

 permeable, and the part where the animals are seen re- 

 ceives simply rain-water. Now the animals seem larger 

 than they were when they lived in warm water, and 

 none are deformed. In this case it seems quite certain 

 that the deformations were the result of the heated 

 water the animals lived in/ and experiments can easily 

 be made to prove the fact, or to disprove it, as the case 

 may be. Ritzema Bos l has observed other deforma- 

 tions, or form-alterations, in very different animals 

 and circumstances. Tylenchi (Tylenchus devastatrix), 

 which have been accustomed for some generations to 



1 Untersuchungen ilber Tylenchus devastatrix. Biol. Centralblatt, 

 vii., 1887, pp. 232-243. 



