FERTILISATION AND THE ORIGIN OF SPECIES. 333 



modifying the same incipient alterations to suit themselves. 

 Hence, as soon as isolation by migration has taken place, it 

 is the presence of other insects which determines the develop- 

 ment of other varieties. All, however, are based on the 

 same plan of departure. 



In this way many varietal and subsequently specific forms 

 of the same genus will arise; and the further they travel 

 from the parental home the greater, perhaps, will be the 

 specific difi"erences; and thus can representative species be 

 accounted for, especially among conspicuously flowering 

 plants. 



On the other hand, the perpetually self-fertilising species 

 whicli alone, as a rule, are cosmopolitan, are almost identical 

 in form, or at least have a minimum of differences between 

 them, and such as may possibly be accounted for by climatal 

 causes alone. 



Difficulties of Natural Selection. — The greatest diffi- 

 culty I have always felt in the idea that a plant was selected 

 because it had some floral structures more appropriate than, 

 others, lay first in the fact that the principal period of the 

 struggle for life takes place in the seedling stage, before any 

 varietal and specific characters have appeared ; and, unless 

 there were a large number of the seedlings which would 

 ultimately bear the improved flower, or else a superior con- 

 stitutional vigour be guaranteed to be correlated with the 

 particular varietal characters to be preserved, these alone 

 could have nothing to do with the survival of the fittest. 



Secondly, granting that the plant has succeeded in sur- 

 viving till the flowering period, then why should so many 

 minute details of floral structure be necessarily correlated ? 

 If the loss of three out of five carpels in the Labiatce w^ere 

 due to natural selection, why should this go band-in-hand 

 with a multiplication of the ribs of the calyx, and the 



