298 GEOLOGICAL BIOLOGY. 



In a previous page are given some of the definitions of 

 species formulated by early naturalists, Alfred R. Wallace, 

 who published as early as 1855 an article on the law which has 

 regulated the introduction of new species (Darwin's '' Origin 

 of Species " was published in 1859), set forth some of the chief 

 principles of the modern evolutionary conception of the his- 

 tory of organisms. Wallace made a careful study of species, 

 and, perhaps as well if not better than any one else, under- 

 stands the relationship between species and geographical dis- 

 tribution. In an article of his " On the Malayan Papilionidae, 

 or Swallow-tailed Butterflies, as Illustrative of the Theory of 

 Natural Selection," published in 1864, is found the following 

 definition of the word species:* 



" In estimating these numbers [of the species of Papilionidfe] I have had 

 the usual difficulty to encounter, of determining what to consider species 

 and what varieties. The Malayan region, consisting of a large number of 

 islands of generally great antiquity, possesses, compared to its actual area, 

 a great number of distinct forms, often indeed distinguished by very slight 

 characters, but in most cases so constant in large series of specimens, 

 and so easily separable from each other, that I know not on what principle 

 we can refuse to give them the name and rank of species. One of the best 

 and most orthodox definitions is that of Pritchard, the great ethnologist, 

 who says that ' separate origin and distinctness of race, evinced by a constant 

 transmission of some characteristic peculiarity of organization,'' constitutes a 

 species. Now leaving out the question of " origin,' which we cannot deter- 

 mine, and taking only the proof of separate origin, 'the constant transmis- 

 sion of some characteristic peculiarity of organization ," we have a definition 

 which will compel us to neglect altogether the amount of difference be- 

 tween any two forms, and to consider only whether the differences that 

 present themselves are permanent. The rule, therefore, I have endeav- 

 ored to adopt is, that when the difference between two forms inhabiting 

 separate areas seems quite constant, when it can be defined in words, and 

 when it is not confined to a single peculiarity only, I have considered such 

 forms to be species. When, however, the individuals of each locality vary 

 among themselves, so as to cause the distinctions between the two forms 

 to become inconsiderable and indefinite, or where the differences, though 

 constant, are confined to one particular only, such as size, tint, or a single 

 point of difference in marking or in outline, I class one of the forms as a 

 variety of the other. I find as a general rule that the constancy of species 

 is in inverse ratio to their range. . . . When a species exists over a wide 

 area, it must have had, and probably still possesses, great powers of dis- 

 persion. . . . When, however, a species has a limited range, it indicates less 

 active powers of dispersion, and the process of modification under changed 



* " Contributions to the Theory of Natural Selection. A Series of Essays." 

 p. 141. Macmillan & Co., 1S70. 



