Ortmann.] J-"^ [May 15, 



This prevention of migration is very important. Migration (as under- 

 stood by M, Wagner) is an accessory factor, often cooperating with sepa- 

 ration, and often working against it. Each species, which originated in 

 a limited area, tends to occupy other territories : it is a well-lsnown fact 

 that each animal form possesses its peculiar "means of dispersal," and by 

 such means it migrates. Migrating species occupy new territories, which 

 have either the same or slightly different conditions of life : in the latter 

 case migration by itself may induce new variations in consequence of the 

 slightly modified action of the external conditions of life. Further, 

 migration is often slow, or only possible under peculiar circumstances, 

 often it is accidental, and only a few individuals can transgress the orig- 

 inal limits on rare occasions : then even migration acts as a means of 

 separation. The few individuals occupying a new locality are afterwards 

 practically separated from the original stock remaining in their native 

 country, and thus they may develop separately into a different species, 

 even in the case that immigration from tlie OHginal stock is not altogether 

 impossible, since any rare individuals of the latter, reaching the new col- 

 ony from time to time, are soon absorbed by the new form and their char- 

 acters disappear J)y the continuous crossing with the modified individuals 

 and by the transfbrming power of the external conditions. Separation, 

 however, is not always connected with migration : the original "centre 

 of origin " of a species may be broken up again into parts, thus inducing 

 the origin of new species, if the external conditions favor it. 



Separation in any form may be more or less complete, and since between 

 complete continuity and complete separation intermediate steps are inter- 

 posed, also a complete differentiation of species is reached by degrees. 

 Tiiis corresponds exactly with what we see in nature. We know of 

 many groups, the species of which are very insufficiently limited and pass 

 gradually into each other : in such cases the formation of species is not 

 yet accomplished. It is an incomplete separation, if a species occupying 

 a large area is divided into different varieties, which are locally more or 

 less limited, and differ in most remote localities considerably, while in 

 intermediate places intermediate forms are present. The distinct varieties 

 on the most extreme limits of the range are certainly under different con- 

 ditions of life, but in the intermediate area transitions are present : a com- 

 plete difterentiation of species is not yet reached here, and we have to 

 regard these forms still as varieties. 



Of course, it is possible, that nearly allied species, which originated 

 separately, may occupy by migration the same territory and come into 

 competition with each other. If their morphological and physiological 

 peculiarities are not sufficiently fixed, there may result by hybridization a 

 new species. But if llie characters are well fixed by iniieritance, espe- 

 cially if there is "kyesamechania," they may live together or the stronger 

 may suppress the weaker. But I may safely say, that it is very improbable 

 that two closely allied species ever lived precisely under the same condi- 

 tions in the same locality. I refer in this respect to the example of four 



