FREE BOOKS

Author's List




PREV.   NEXT  
|<   85   86   87   88   89   90   91   92   93   94   95   96   97   98   99   100   101   102   103   104   105   106   107   108   109  
110   111   112   113   114   115   116   117   >>  
type, though why this should have happened does not appear to be clear. At the same time those specimens which tended to vary in the direction of _A. echeria_ in places where this species was more abundant than _A. dominicanus_ were encouraged by natural selection, and under its guiding hand the form _mima_ eventually arose from _wahlbergi_. According to Mendelian views, on the other hand, {146} _A. echeria_ arose suddenly from _A. dominicanus_ (or _vice versa_), and similarly _mima_ arose suddenly from _wahlbergi_. If _mima_ occurred where _A. echeria_ was common and _A. dominicanus_ was rare, its resemblance to the more plentiful distasteful form would give it the advantage over _wahlbergi_ and allow it to establish itself in place of the latter. On the modern Darwinian view natural selection gradually shapes _wahlbergi_ into the _mima_ form owing to the presence of _A. echeria_; on the Mendelian view natural selection merely conserves the _mima_ form when once it has arisen. Now this case of mimicry is one of especial interest, because we have experimental evidence that the relation between _mima_ and _wahlbergi_ is a simple Mendelian one, though at present it is uncertain which is the dominant and which the recessive form. The two have been proved to occur in families bred from the same female without the occurrence of any intermediates, and the fact that the two segregate cleanly is strong evidence in favour of the Mendelian view. On this view the genera _Amauris_ and _Euralia_ contain a similar set of pattern factors, and the conditions, whatever they may be, which bring about mutation in the former lead to the production of a similar mutation in the latter. Of the different forms of _Euralia_ produced in any region that one has the best chance of survival, through the operation of natural selection, which resembles the most plentiful _Amauris_ form. Mimetic resemblance is a true phenomenon, but natural selection plays the part of a conservative, not of a formative agent. [Illustration: PLATE VI.] {147} It is interesting to recall that in earlier years Darwin was inclined to ascribe more importance to "sports" as opposed to continuous minute variation, and to consider that they might play a not inconsiderable part in the formation of new varieties in nature. This view, however, he gave up later, because he thought that the relatively rare sport or mutation would rapidly disappear through the swamping effec
PREV.   NEXT  
|<   85   86   87   88   89   90   91   92   93   94   95   96   97   98   99   100   101   102   103   104   105   106   107   108   109  
110   111   112   113   114   115   116   117   >>  



Top keywords:
selection
 

natural

 

wahlbergi

 
echeria
 
Mendelian
 
mutation
 

dominicanus

 

resemblance

 

suddenly

 

Amauris


Euralia
 
similar
 

plentiful

 

evidence

 

operation

 

survival

 

chance

 

region

 

resembles

 

conservative


formative
 

Mimetic

 

produced

 
phenomenon
 

factors

 
conditions
 
pattern
 

happened

 

production

 

Illustration


nature

 

varieties

 
inconsiderable
 
formation
 

disappear

 
swamping
 

rapidly

 

thought

 

earlier

 

Darwin


recall

 

interesting

 
genera
 

inclined

 
ascribe
 
minute
 

variation

 

continuous

 
opposed
 

importance