290 THE THEORIES OF EVOLUTION 



which chiefly distinguish species, do not result from 

 natural selection, but from some form of geograph- 

 ical isolation and the segregation of individuals re- 

 sulting from it." 4 



We must add that, in order to test the accuracy of 

 his own deductions, Dr. Jordan sent to a number of 

 American ornithologists a question blank asking them 

 among other important questions whether two or 

 more subspecies had ever been observed to inhabit 

 exactly the same region. The answers were unani- 

 mously negative, thus upholding Dr. Jordan's con- 

 tention. 



Another form of isolation is physiological isolation, 

 or physiological selection, as it has been termed by 

 Romanes, who gives it much credit for species-form- 

 ing. One can observe, he thinks, at a given time 

 within a given species, variations which make inter- 

 generating among all the individuals impossible and 

 restrict it to a certain category only, variations in the 

 structure of the genitalia, differences in the epochs 

 of maturation of the genital product, modifications 

 in the sexual instinct, etc. Romanes cites in support 

 of his theory the data collected by Gulick and the ob- 

 servations made by the botanist A. Jordan on plant 

 varieties which, while presenting only insignificant 

 morphological differences, give generally negative re- 



4 The Origin of Species through Isolation. (Science, Nov. 3, 1905, p. 

 557.) 



