128 ANALYSIS OF THE FOUR PRINCIPLES (CONTINUED). 



6. Fertilizational Isolation. 



Francis Gal ton's, short article on "The Origin of Varieties," which 

 was published in Nature, vol. xxxiv, p. 395, refers to this cause of 

 isolation. He says : 



If insects visited promiscuously the flowers of a variety and those of the parent 

 stock, then — supposing the organs of reproduction and the period of flowering to 

 be aUke in both, and that hybrids between them could be produced by artificial 

 cross-fertilization — we should expect to find hybrids in abundance whenever 

 members of the variety and those of the original stock occupied the same or 

 closely contiguous districts. It is hard to account for our not doing so, except on 

 the supposition that insects feel repugnance to visiting the plants interchangeably. 



It is evident that isolation of this form depends on divergence of 

 character already clearly established, and, therefore, on some other 

 form of isolation that has preceded. It is also segregative rather than 

 separative, in that it perpetuates a segregation previously produced, 

 which might otherwise be obliterated by the distribution of the differ- 

 ent forms in the same district. The form of isolation that precedes 

 fertilizational isolation, producing the conditions on which it depends, 

 must, in the majority of cases, be local isolation. Chronal and 

 impregnational isolation, when imperfectly established, might be for- 

 tified by fertilizational isolation, but, in the case of plants, these are 

 usually dependent on previous local isolation. 



7. Artificial Isolation. 



Artificial isolation is isolation arising from the relations in which 

 the organism stands to the rational environment. 



The importance of environal isolation. — We must not assume that 

 the various forms of environal isolation are of small influence in the 

 formation of species because sexual or impregnational incompatibility 

 is a more essential feature, without which all other distinctions are 

 liable to be swept away. The importance of the environal forms of 

 isolation lies in the fact that they often open the way for the entrance 

 of the more fundamental forms of segregation, even if they are not 

 essential conditions for the development of the same. Though 

 myriads of divergent forms produced by local and industrial isolations 

 are swept away in the struggle for existence, and myriads are ab- 

 sorbed in the vast tides of crossing and intercrossing currents of life, 

 the power of any species to produce more and more highly adapted 

 variations, and to segregate them in groups that become specially 

 adapted to special ends, or that grow into specific forms of beauty and 

 internal harmony, is largely dependent on these factors. 



