88 EXPERIMENTAL EVOLUTION LECT. 



of known influence. It is quite natural that exercise 

 should have an influence upon growth and development, 

 and that in cases where there are physiological or 

 mechanical impedimenta to movement, dwarfing should 

 be the result. I think that this is the explanation 

 which must be accepted ; and if animals living in 

 confined spaces remain small, this is due to the fact 

 that they cannot move enough. At all events Prof. 

 Semper's interpretation seems to me not acceptable. 

 Further experiments will yield new facts, and time 

 will tell whether this explanation is sufficient 



In this connection I may call attention to the cir- 

 cumstance that an observer who has given some study 

 to dwarfing in Lymncea has pointed out a singular 

 fact connected with this process, but one which 

 requires to be confirmed by new investigations. 



It is the fact that dwarfed forms are generally 

 exclusively female, and that their liver offers a con- 

 siderable amount of degeneration. 



So much for dimensional variation. 



If we now pass on to consider the integument, we 

 perceive that in this part, and in its appendages, 

 variations are numerous and also important. Many 

 animals, when transferred to warm climates, lose 

 their wool, or their hairy covering is much reduced. 

 In some parts of the warmer region of our earth 

 sheep have no wool, but merely hairs like those of 



