104 The Scottish Naturalist. 



6. Migratory Species. — It seems unnecessary to say anything in 

 explanation of this section. 



7. Variations of species in the District. — The variations that 

 species are subject to, form several distinct classes, and in con- 

 nection with the theory of evolution, are of much importance. It 

 is, therefore, desirable not only that the variations that occur within 

 our bounds should be studied, but that a definite method of re- 

 cording their occurrence should be adopted. The classification ot 

 forms which depart from the type of the species is not altogether 

 easy. What some naturalists consider to be a good and distinct 

 species, is considered by others to form only a sub-species, or even 

 to be unworthy of the latter rank. If, therefore, so much difficulty 

 attends the definition of a species, there is at least as much in de- 

 fining the grades of the variations. 



Beginning with the species itself, the following are the more im- 

 portant grades : — 



1. The Species (or super-species). 



2. The Sub-Species. 



3. The Local Race, or Variety. 



4. The Aberration. 



5. The Abnormality. 



In addition to these, cases are often met with presenting slight 

 differences, which, being almost intangible, are difficult to charac- 

 terise, but which as showing possibly the beginnings of, or the re- 

 versions from, more important variations, are worthy of careful 

 study. 



Many species are ordinarily so fairly constant and uniform in 

 colour and structure, that they may be said to offer no variations 

 from the type or characteristic form unless it be an occasional ab- 

 normality. By an abnormality is meant a departure from the 

 characters of the type sufficiently great to be more or less notice- 

 able, but which is of such rare occurrence that it may perhaps be 

 observed once only and never again. An albino form (in the true 

 sense) is an abnormality, but it may take other aspects. Between 

 the aberration and the abnormality there is often only the degree 

 of a greater frequency of recurrence. That is to say, the abnor- 

 mality, by showing a tendency to become perpetuated, or at least 

 to recur not unfrequently, becomes an aberration. The term 

 "variety" is often used in the same sense as we have used ■' aber- 

 ration," but may more conveniently bj applied to the next grade 



