248 THE POPULAR SCIENCE MONTHLY. 



environment slow enough to he accompanied by change of the 

 species de^Dending upon it. There can he but little doubt that 

 many if not most or all of existing s^jecies are slowly changing 

 in mass and in place into something else than what they are 

 now. This process may be so slow that it will require some such 

 careful study and delicate calculation as has been used by as- 

 tronomers to prove that the so-called fixed stars are in motion ; 

 but the well-founded belief that geological changes are still go- 

 ing on upon the earth is enough to make us certain that changes 

 ' in living things must accomjjany these or follow close after. 



In addition to such slow changes of the mass of individuals 

 of a species as must finally produce a form specifically different 

 from it, there is another method of formation of species which 

 would seem to be much more prolific in its results, and which 

 alone can keep the earth populated during ages of changes of 

 environment, some of them so rapid and violent that they destroy 

 great numbers of species. This is by the migration of individuals 

 into surrounding areas where the conditions are so near like 

 those to which they are accustomed that they can exist, and still 

 different enough to set up rapid changes of structure. The off- 

 shoots of the parent species might thus become numerous, and 

 still retain likeness enough to each other so that they would be 

 thrown by the systematist into a common genus; or, if the 

 changes of environment were less, we should have a set of sub- 

 species or varieties. There is no line or rule fixed in nature by 

 which we can say that this is a genus with several species, and 

 this is a species with several varieties. If this is a true theory 

 for the creation of species, there should exist certain species 

 settled and fixed in character, which have existed with but the 

 slow modifications of structures caused by geological changes, 

 while other species would be ready to change in any direction, or 

 to revert rapidly to the characters of its parent species. The 

 varieties and species of man's make are exaggerated examples of 

 the latter class, and man himself would furnish illustrations of 

 the same thing in such ancient and well-established species as 

 the Chinese, and such a recently-formed variety as the Anglo- 

 American. The heterogeneous character of the conditions of 

 environment bearing upon life, and their utter lack of equality 

 or equivalence of modifying power, give good and sufficient rear 

 son for that lack of equality of structural values among groups, 

 which is best shown by the inability of the best systematists to 

 agree upon their value. The fate of the group called species is a 

 case in point. As long as the animals of a single well-defined 

 area were studied, there was no difficulty, for closely-allied species 

 necessarily rarely or never inhabit the same area ; but as soon as 

 the study became comparative, by bringing in forms from neigh- 



