HISTORY OF ZOOLOGY 43 



character of common pigeons. Finally, it has been claimed that for the 

 formation of new species a simple variation of form is not sufficient; it 

 must reach still farther: (i) a variation in different directiojis, a divergent 

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

 of the transitional forms which unite the divergent forms. 



The objection that the struggle for existence cannot bring about the 

 divergent development of individuals necessary for improvement is of 

 least importance. Of the many variations appearing at the same time in a 

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

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

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

 quently 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 difl'erentiated com- 

 panions of their species, and, being less completely adapted, must fall. 



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

 that the principle of selection alone is insulhcient to explain the origin 

 and adaptive modification of new forms. So new theories have been 

 advanced, older ones revamped, sometimes to replace that of natural 

 selection, sometimes to strengthen certain links of its chain. Limited 

 space permits only an outline of the more prominent of these and that 

 without any attempt at a decision as to how far they complete the Darwin- 

 ian hypothesis, are compatible with it, or replace it. 



Germinal Selection. — According to its author, Weismann's 'germinal 

 selection ' is only a completion of natural selection, the 'individual selection.' 

 It presupposes a detailed knowledge of modern investigations in the lines 

 of fertilization and heredity (see chapter on Fertilization) and hence can 

 only be outlined here. Weismann believes that all variations which are 

 selected in the struggle for existence and are fixed in the successive 

 generations must have their sources in the germ cells and since these arise 

 from the fusion of male and female sex cells, they must, in the first in- 

 stance, have been contained in these. Each germinal anlage consists of 

 extremely numerous elements, the 'determinants' of the peculiarities 

 of the organism. Accordingly as certain of these determinants develop 

 at the expense of the others or are weakened or modified, the organism 

 arising from the germ has special peculiarities or variations. If certain 

 determinants tend constantly in a certain direction and these be present 

 in large numbers, these will persist so that individual selection can have 

 its influence. 



Mutations. — The Mutation theory of de Vries is a considerable 

 modification of the Darwinian theory. In rearing large numbers of the 



