THE 
New Phytologist 
Vol. XIX, Nos. 9 & io. Nov. & Dec., 1920 . 
[Published January, 1921.] 
MUTATIONS AND EVOLUTION. 
By R. Ruggles Gates. 
CHAPTER VII. 
Mutations in Animals. 
I N animals, the evidence for polymorphism arising through 
mutation is equally extensive, and a study of any group 
systematically reveals innumerable instances. As with plants, 
many of the forms arising in this way, especially in conditions of 
captivity where selection is eliminated, are to be classed as 
abnormal, but there are many others to which this does not apply. 
They represent innocuous or occasionally even advantageous 
changes, or changes which would be advantageous in certain 
environments. * 
One of the convincing cases of an expanding polymorphic 
group whose various types are mutational in origin is to he found 
in the land-snails. Gulick (1905) studied the Achatinellidae of 
Hawaii, and the consideration of isolation as an evolutionary 
factor is frequently based on his work. More recently Crampton 
(1917) has investigated the related land-snail genus Partula of 
the Society Islands. It is well known that each valley often has 
its own peculiar species or varieties of snails, and this is the 
classical case of evolution under isolation. From the later studies 
it has become clear that isolation is not a factor of evolution but a 
condition in which evolution by mutation may take place. 
Crampton’s extensive study of the genus Partula in Tahiti has 
led to definite and well-subtantiated conclusions. Over 80,000 
individuals were collected during visits in the years 1906 to 1909. 
Accurate records of the distribution of species and varieties had 
been made by Garrett in 1884, and it was thus possible to determine 
what changes in the snail fauna had taken place during the inter¬ 
vening 25 years. During this period a number of new forms have 
