62 GENERAL PRINCIPLES OF ZOOLOGY. 



the ordinary; the muscles for moving it, the supporting 

 skeletal parts, the nerves running to it must have a definite 

 formation and arrangement ; in short, there must exist a 

 harmonious co-operation of parts, which presupposes a pro- 

 gressive and well-regulated development extending through 

 a long space of time during which the struggle for exist- 

 ence could have exerted no directing influence. A further 

 objection is that the action of natural selection would 

 under ordinary conditions be compensated by the swamp- 

 ing effect of unhindered crossing of the varying forms. If, 

 for example, we do not isolate fantails from other pigeons, 

 they will cross with these, and their descendants will soon 

 take on 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: (i) a variation in different directions, a divergent 

 development of the individual constitution of a species; 

 (2) the disappearance of the transitional forms, which unite 

 the divergent species. 



The objection that the 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 appear- 

 ing at the same time in a species two or more may be 

 equally useful ; that 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 dif- 

 ferent directions. Consequently the intermediate forms 

 which are not especially stamped 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 adapted, must yield to them. 



More important are the two first-named objections; 

 they have led to theories which originally seemed destined 

 to complete the Darwinian theory, but in the course of dis- 

 cussion they have more and more raised the claim of entirely 

 supplanting it. In the following paragraphs will be found 



