GEOGRAPHIC ISOLATION AND SPECIES-FORMING 123 



have been factors in their formation. We claim that such 

 conditions are virtually universal among species as they exist 

 in nature. When the geographical relations of the origin of a 

 species cannot be shown it is usually because the species has not 

 been critically studied, from absence of material or from absence 

 of interest on the part of naturalists. In a few cases, a species 

 ranges widely over the earth, showing little change in varying 

 conditions and little susceptibility to the effects of isolation. 

 In other cases, there is some possibility that saltations, or 

 suddenly appearing characters, may give rise to a new species 

 within the territory already occupied by the parent form. 

 But these cases are so rare that in ornithology, mammalogy, 

 herpetology, conchology, and entomology, they are treated as 

 negligible quantities. 



One of the most successful workers in this field is Rev. John 

 T. Gulick, whose studies of the localization of species and sub- 

 species of land snails in Oahu Island (Hawaii) have become 

 classical. According to Mr. Gulick, the land snails of the 

 wooded portion of Oahu have become split up into about 

 175 species of land shells represented by 700 or SCO varieties. 

 He frequently finds a genus represented in several successive 

 valleys by allied species, sometimes feeding on the same and 

 similar plants. In every case, the valleys that are nearest to 

 each other furnish the most nearly allied forms, and a full set 

 of the varieties of each species presents a minute gradation 

 between the more divergent types found in the more widely 

 separated localities. Similar conditions are recorded among 

 the land snails in Cuba and in other regions. In fact, on a 

 smaller scale, the development of species of land and river 

 mollu.sks has everywhere progressed on similar lines with that 

 of birds and fishes. To recognize isolation as practically a 

 necessary condition in the subdivision of species need not 

 necessarily eliminate or belittle any other factor. Isolation is 

 a condition, not a force. Of itself it can do nothing. Species 

 change or diverge with space and with time: with space, be- 

 cause geographical extension divides the stock and brings 

 new conditions to part of it; with time, because time brings 

 always new events and changes in all environment. The 

 beginning of each species must rest with its variability of 

 individuals. 



One of the most remarkable cases of group evolution is 



