CH. Ill] INDEPENDENT ORIGIN OF ALLIED FORMS. 133 



with modification we are in a better position to grapple 

 with the problems offered by the phenomena of the 

 distribution of animals. Indeed if we hold to the opposite 

 theory there are no problems for discussion. 



But though it is absolutely necessary to believe in 

 some theory of descent and modification in order to 

 explain the facts of distribution, this theory itself presents 

 some difficulties which will be briefly indicated. It is 

 usually held that a given species can only come into 

 existence once ; that the same modification can only 

 appear once and never again. Consequently if we meet 

 with the same species in two separate localities, there 

 must have been some time or other communication of 

 some kind between them ; either the animal in question 

 has been able to traverse the intervening barrier or the 

 barrier at one time did not exist. If the opposite view 

 be maintained, and there are some evolutionists who have 

 maintained it, many of the problems connected with 

 distribution will at once disappear. Probably a middle 

 course is here as in so many cases the safer one to follow. 

 The degree of complication of the changes is in all 

 likelihood a safe guide to follow. It is for example 

 inconceivable, as Prof. Lankester has pointed out, that 

 animals which agree in the possession of that characteristic 

 and complicated organ peculiar to the Mollusca and 

 known as the odontophore should not be all of them 

 genetically related. But on the other hand it is easily 

 conceivable that two birds might independently lose a 

 certain muscle ; the fact therefore that they were both 

 without this particular muscle would not be an infallible 



