RELATIONSHIP OF SPECIES. 249 



species previously horned (individual non-merging character- 

 istics) as mutations, such evolutions as those of racial and 

 merging characteristics one may count to be either variation or 

 mutation, according as the difference is great or small, but with 

 an indefinable separation line ? In short, that there is but one 

 evolutionary process at work, and that the rate at which this 

 progresses must of necessity (to be in harmony with a more 

 or less urgent environment) be of varying magnitudes, and 

 hence may lead us into classifying these results as separate 

 phenomena. 



It may be of interest to mention the phenomenon of Inde- 

 pendent Evolution, a possibility somewhat unconsidered. Dr. 

 Lloyd, discussing the origin of the brown white-bellied type 

 of Mus rattus* surmises that they arise independently all over 

 India, and are not due to one evolution which has migrated. 

 I should imagine that this is a very reasonable deduction from 

 the facts he mentions. I have at present some specimens of 

 this brown type in my possession which were bred by Miss 

 Frances Pitt t from black parents. It is impossible, however, to 

 say if these were an original mutation, or whether mutation 

 occurred previously at the place where the parents were caught 

 (a floating hulk), or in the district in general (Plymouth). 

 Moreover, a seaport is hardly a place to expect isolation. The 

 matter, however, is interesting if only from the fact that this 

 brown type is held to be specifically distinct, under the name 

 M. alexandrinus. 



A more trustworthy example of types separately evoluted is 

 one given previously in connection with Leptinotarsa rubicunda, 

 this type being produced independently from L. multitceniata, 

 and, while sterile with its parent form, breeding congenially with 

 a similar identical though unrelated (genealogically) species. 



As groups or species appear to arise thus in several places, 

 it follows it is not impossible that genera or families may do 

 likewise, that is, the process might be continued, pari passu, to a 

 greater extent, and this might account for curious geographical 

 distribution, such as that of the Tapirs. Platyrrhine and 

 Catarrhine Monkeys may be more widely separated than many 



* ' The Growth of Groups.' 



•f- ' National Review,' vol. lix. p. 862. 



