VARIOUS THEORIES OF SPECIES-FORMING 109 



But it is conceivable that isolation may be effected in other 

 ways than by actual segregation or geographic separation of 

 individuals. Anything that could lead to exclusive or dis- 

 criminate breeding among certain individuals of a species would 

 result in the isolation of these individuals from the rest of the 

 species as effectively as their actual separation from others by 

 a geographic or topographic barrier. Now there are various 

 influences or conditions that might conceivably bring about 

 such a state of affairs, and some of these have been actually 

 observed to exist. It is of interest to note that this kind of 

 isolation differs, in a rather important way, from purely geo- 

 graphic isolation in that the latter is almost sure to be wholly 

 indiscriminate as regards the individuals comprised in an isolated 

 group, while the former, which has been called physiological 

 isolation, will be discriminate. That is, there will be a struc- 

 tural or physiological peculiarity common to all the " isolated ' 

 individuals, it being by virtue of this common peculiarity 

 (something not common to other individuals of the same 

 species) that the isolation actually exists. 



Romanes has been the chief champion of the physiological 

 isolation factor. And we may advantageously refer directly to 

 his writings for a specific statement of different forms or phases 

 of this kind of isolation. In "Darwin and After Darwin," III, 

 p. 7 et seq., he writes: 



"Now the forms of discriminate isolation, or homogamy, are very 

 numerous. When, for example, any section of a species adopts 

 somewhat different habits of life, or occupies a somewhat different 

 station in the economy of nature, homogamy arises within that section. 

 There are forms of homogamy on which Darwin has laid great stress, 

 as we shall presently find. Again, when for these or any other reasons 

 a section of a species becomes in any small degree modified as to form 

 or color, if the species happens to be one where any psychological pref- 

 erence in pairing can be exercised as is very generally the case among 

 the higher animals exclusive breeding is apt to ensue as a result 

 of such preference; for there is abundant evidence to show that, both 

 in birds and mammals, sexual selection is usually opposed to the 

 intercrossing of dissimilar varieties. Once more, in the case of plants, 

 intercrossing of dissimilar varieties may be prevented by any slight 

 difference in their seasons of flowering, of topographical stations, or 

 even, in the case of flowers which depend on insects for their ferti- 



