ORGANIC FORM AND ARCHITECTURE 7x3 



group so distinctive that it deserves a name all to itself. Most 

 ornithologists believe that the Red Grouse, a bird peculiar to 

 Britain (apart from introductions elsewhere), is derived from the 

 Willow Grouse, at home in Scandinavia, but no one believes that a 

 Willow Grouse turned into a Red Grouse. 



The first step in the origin of a new species is the emergence of 

 variations or mutations, whose causes are still very obscure. That 

 is a separate problem— the origin of variations or mutations, pre- 

 sumably from some shuffling of the hereditary cards or from some 

 deep change in the constitution of the germinal protoplasm. These 

 variations are very common, and it is not for our present purpose 

 begging any question to take them as given. 



A second step in the evolution of a new species b the inbreeding 

 of similar variants. If the same new departure has been exhibited 

 about the same time by a number of individuals, these may inter- 

 breed, and this will tend to increase the ranks of the contingent 

 bearing the new features. If a variant does not find a similar variant 

 with which it can breed, it may pair with a member of the original 

 stock; and if the novel character is a Mendelian "dominant" to its 

 absence or its counterpart, the offspring will all resemble the variant 

 in that respect ; and in the next (second filial) generation the number 

 of individuals bearing the novel feature or features will have con- 

 siderably increased. The more the similar variants interbreed, the 

 more fixed and widespread will the new feature become. Thus there 

 is the beginning of a new race or variety. In this way some strong 

 breeds of domesticated animals and races of cultivated plants have 

 arisen from a few original variants or even from one. 



Another factor in the making of a species is isolation, a general 

 term for the various ways in which the range of inter-crossing is 

 lessened. One of the simplest of these ways is literal insulation, for 

 when a new departure occurs on an island, or on a peninsula that 

 becomes an island, there will be more likelihood of similar forms 

 pairing together. Some Field Mice {Apodemus sylvaticus), the com- 

 monest mammals in Europe, seem to have been carried as stowa- 

 ways on a fishing smack from Scotland to the Fair Isle off Shetland. 

 They flourished and multiplied there and a new departure arose, 

 which was helped by its insulation to become a successful race, and 

 by and by a new species, A. fridariensis. This is, in any case, a 

 reasonable interpretation of what occurred. Of recent years some 

 of the Fair Isle Field Mice seem to have been introduced into the 

 Island of Foula, about sixteen miles out in the Atlantic to the west 

 of Shetland; and there the story has repeated itself. For now a new 

 sub-species, A. fridariensis thuleo, has been established. But rela- 

 tive isolation may occur in other ways, e.g. by a change in the 

 course of a river, by alterations in land level, by differences in the 

 time of breeding, by divergences in habit, by the discovery of 



