52 GENERAL PRINCIPLES OF ZOOLOGY. 



radially S3rmmetrical, many in addition segmented. In all these 

 cases tlie same organ is repeated two or more times. Organs which, 

 are repeated symmetrically and usually those which are segmental 

 agree in general in structure. One must therefore admit that the 

 alterations of chance must have occurred at at least two points 

 simultaneously and in exactly the same way. 



A further oljjection is that the action of natural selection would 

 under ordinary conditions be negatived by unhindered crossing of 

 tlie varying forms. If, for example, we do iiot isolate fantails from 

 other jiigeons, they will cross with these, and their descendants 

 will soon resume the character of common pigeons. Finally, it 

 has been claimed that for the formation of new species a simple 

 variation of forms is not sufficient; it must reach still farther: (1) 

 a variation in different directions, a divergent development of the 

 individual members of a species; (2) the disappearance of the 

 transitional forms whicli unite the divergent forms. 



The objection that tlie struggle for existence cannot bring 

 about the divergent development of individuals necessary for 

 improvement is of least importance. It need only be added that 

 of the many variations appearing at the same time in a species two 

 or more may be equally useful; that then one set of individuals 

 will seize Tipon one, another set upon the other advantage, and that 

 in consequence of this both sets will develop in different directions. 

 Consequently the intermediate forms which are not pronounced in 

 the one or the other direction will be in an unfavorable position, 

 and must carry on the struggle for existence with both groups of 

 partially differentiated companions of their species, and, being less 

 completely adap)ted, must fall. 



More important are the first two objections; they have led to 

 theories which originally seemed destined to complete the Dar- 

 winian theory, but in the course of discussion they liave more and 

 more raised the claim of entirely supplanting it. In the following 

 paragraphs will be found an outline of these theories, but it is to 

 be taken into consideration that, at the present time, we are still 

 in the midst of the reform movement, and it cannot yet be said 

 whether they will be able to stand beside the theory of the struggle 

 for existence or will supplant it. 



Migration Theory.— To explain how characters newly formed 

 by variation become fixed, and do not disappear again through 

 crossing with differently modified individuals. M. Wagner has pro- 

 posed tlie Theory of Geographical Isolation, or the Migration 

 Theory. New species may arisf if a part of the individuals of one 



