310 



SCIENCE. 



[N. S. Vol. XXIII. No. 582. 



August 18 to October 23; C. compactus, 

 August 26 to October 19. The times of flight 

 of the three species nearly coincide and they 

 are competitors in the order named by 80 per 

 cent., 66 per cent, and 45 per cent, of their 

 observed visits. According to the views ex- 

 pressed here, three closely related species hav- 

 ing the same habits would not be expected to 

 originate in the same neighborhood. A spe- 

 cies having an abundant food supply will 

 simply increase in the number of individuals. 

 The three species above mentioned are not 

 closely related. They have evidently become 

 competitors by migrating from the outside. 



In Andrena there are three species, each an 

 oligotropic visitor of willows and each having 

 a form, or closely related species, in which 

 the female has the abdomen red. At least as 

 far as these species go, it will refute my view 

 if it can be shown that the red form indicates 

 the development of a new species having the 

 same habits and the same range. I regard the 

 red form as a southern geographical race, or 

 species, and hold that the forms in their dis- 

 tribution merely overlap here. 



The views here stated may be expressed in 

 the following propositions: 



1. To occupy the same ecological position 

 two species must have the same geographical 

 and phenological range and the same food 

 habits. 



2. No ecological position is favorable for an 

 unlimited number of individuals. 



3. The origin of new species results from 

 the fact that the dominant species produce 

 more individuals than can occupy the same 

 position. 



4. Natural selection then operates in favor 

 of any set of individuals which changes hab- 

 itat or habits so as to avoid competition with 

 the dominant form. 



5. The dominant form retains the original 

 position. 



6. The new form becomes specialized in ad- 

 justment to the new position. 



7. The least specialized members of a group 

 occupy the original position. The special- 

 ized members of a group have not driven 

 out the original forms from the original posi- 

 tion, but have been driven out by them. The 



highest specialized members are the ones 

 which have most frequently been driven out 

 of their positions by the competition of lower 

 forms. 



8. Specific characters usually are not adapt- 

 ive. 



9. Specific characters are the result of the 

 intercrossing of the members of an ecological 

 segregate and are the result rather than the 

 cause of the segregation. 



10. Adaptation to a position is determined 

 by the nature of the position rather than by 

 the characters fitting the organism for it. 

 Usually it does not require the development 

 of adaptive characters and usually is not asso- 

 ciated with them. 



11. Adaptive characters are the result of the 

 operation of natural selection after the eco- 

 logical segregation takes place and do not 

 precede the occupation of the new position. 



12. An ecological position is more favorable 

 to a limited number of individuals with im- 

 perfect adaptive structures than to a great 

 number of individuals having more perfect 

 structures. 



13. An ecological basis for morphology is 

 found in the change ef habits which requires 

 an old organ to be used for a new purpose. 

 An ecological basis for evolution is indicated 

 by the endless taxonomic difficulties resulting 

 from adaptation to function. 



14. Species having the same habits are pro- 

 duced by geographical migration. 



15. Species having different habits are pro- 

 duced by ecological selection. 



Charles Robertson. 



' barriers ' and ' bionojiic barriers ' ; or isola- 

 tion and non-isolation as bionomic factors. 

 During the last three months there have 

 appeared in Science a most interesting series 

 of communications, contributed by both zool- 

 ogists and botanists, on the influence of isola- 

 tion as a factor in the evolution of species and 

 subspecies. While there has been some dis- 

 agreement as to the facts in the case, especially 

 from the side of the botanists, the zoologists 

 appear to diiler mainly in respect to the appli- 

 cation of terms to phenomena about the exist- 

 ence and relations of which there is practically 



