External and Internal Factors in Evolution 305 



Conversely, many animals when transferred from warm to 

 cold climates acquire a thicker covering ; dogs and horses, for 

 instance, becoming covered with wool." 



A number of kinds of snails that were supposed to belong 

 to different species have been found, on further examination, 

 to be only varieties due to the environment. " Locard has 

 discovered through experiments that L. turgida and elophila 

 are mere varieties — due to environment — of the common 

 Lymncea stagnalis." He says, "These are not new species, 

 but merely common aspects of a common type, which is 

 capable of modification and of adaptation according to the 

 nature of the media in which it has to live." It has also 

 been shown by Bateson that similar changes occur in 

 Cardium edule, and other lamellibranchs are known to vary 

 according to the nature of the water in which they live. 



In regard to plants, the influence of the environment has 

 long been known to produce an effect on the form, color, 

 etc., of the individuals. " The common dandelion ( Taraxa- 

 cum densleonis) has in dry soil leaves which are much more 

 irregular and incised, while they are hardly dentate in marshy 

 stations, where it is called Taraxacum, palustre. 



" Individuals growing near the seashore differ markedly 

 from those growing far inland. Similarly, species such as 

 some Ranunculi, which can live under water as well as in 

 air, exhibit marked differences when considered in their 

 different stations, as is well known to all. These differences 

 may be important enough to induce botanists to believe in 

 the existence of two different species when there is only 

 one." 



An interesting case is that of Daphnia rectirostris, a small 

 crustacean living sometimes in fresh water, at other times 

 in water containing salt and also in salt lakes. There are 

 two forms, corresponding to the conditions under which they 

 live, and it is said that the differences are of a kind that 

 suffice to separate species from each other. In another 



