128 ANALYSIS OF THE FOUR PRINCIPLES (CONTINUED). 
6. Ferttlzational Isolation. 
Francis Galton’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 alike 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 tsolation.—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. 
