DISCONTINUITY OF SPECIES. 35 



subject to them on the whole form a discontinuous scries. * * * Tempera- 

 ture, altitude, depth of water, salinity, in fad most of the elements which make 

 up the physical environment arc continuous in their gradations, while as a rule 

 the forms of life are discontinuous. Besides this, forms which are apparently 

 identical live under conditions which arc apparently very different, while species, 

 which though closely allied are constantly distinct, arc found under conditions 

 which are apparently the same. 



He suggests that the explanation of this lack of correspondence 

 must be sought in the organic group, and not in its environment; and 

 that in the study of variation is the chief hope though even that may- 

 fail (p. 17). 



I entirely agree with Mr. Bateson in regard to the importance of 

 variation and of the factors in the species that control variation ; and 

 prominent among these factors I find cither the power of free com- 

 munication and intergeneration or the lack of this power. When a 

 species possessing very limited powers for migrating and very rare 

 opportunities for transportation is surrounded by a mass of species 

 having the usual powers and opportunities, the natural result is that 

 many colonies from the species having very limited powers become 

 completely isolated from each other and from the original stock, 

 while the surrounding species of plants and animals are not isolated 

 from the groups of the same species in other places. A further result 

 is that the isolated colonies become divergent, while the species hav- 

 ing powers that prevent isolation remain the same as in the original 

 home. The isolated groups being prevented from crossing with each 

 other, there is nothing to prevent each group from establishing its 

 own special methods of dealing with the environment, and thus sub- 

 jecting itself to special forms of selection, though dealing with the 

 same environment. No better examples can be found of the diver- 

 gence of isolated groups while exposed to the same environment than 

 those presented by the arboreal snails of the Hawaiian Islands and of 

 some other regions. 



The probabilities are completely reversed in the case of a species 

 possessing unusual powers for migrating, or extraordinary opportu- 

 nities for transportation. For it often happens that groups of such a 

 species, occupying districts very unlike, not only in climate but in the 

 prevalent species of plants and animals, will maintain free communi- 

 cation and intergeneration with each other, and thus be kept to 

 essentially one type. Good examples of unity of type maintained 

 notwithstanding long exposure to diverse conditions are found in the 

 case of certain species of birds and insects possessing great powers of 

 flight. 



