Lixternal 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. ¢wrgida and elophila 
are mere varieties—due to environment— of the common 
Lymnea 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 (7arara- 
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 Zavaxracum palustre. 
“Tndividuals 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 
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