72 Mutations and Evolution. 



CHAPTER VII. 

 MUTATIONS IN ANIMALS. 



IN 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 be 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-substantiated 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 



