HISTORY OF ZOOLOGY. 51 



is shown, for example, in the case of the birds of paradise 

 (fig. 15) ; for the nesting female inconspicuous colors and a close- 

 lying coat of feathers are necessary in order that, undisturbed by 

 enemies, she may devote herself to incubation. 



On the Efficiency of Natural Selection. In the course of the 

 last decade there has been much controversy as to how far natural 

 selection alone is a species-forming factor. A number of objectors 

 dispute the possibility of fortuitous variations being utilized in the 

 struggle for existence. It is not easy to see how many characters, 



FIG. 15s. Paradisea apoaa, female. (After Levaillant.) 



especially such as are used in classification, can be of use to their 

 owners. It can only be said that they have developed in correla- 

 tion, that is in necessary organic connexion, with other important 

 characters. But useful characters must be considerable in order 

 to be seized upon by natural selection. Fortuitous variations with 

 which Darwinism deals are too inconsiderable to be utilized by the 

 organism and so to be of value in the struggle for existence. In. 

 most cases, too, alteration in one organ alone is not enough to be 

 of value; usually a whole series of accessory structures must be 

 modified. In short, there must exist a harmonious co-operation 

 of parts, which presupposes a progressive and well-regulated 

 development extending through a long space of time during which 

 the struggle for existence could have exerted no directing influ- 

 ence. Thus, for example, the wing of a bird in order to be used 

 for flight must have already reached a considerable size; the 

 muscles for moving it, the supporting skeletal parts, the nerves 

 running to it must have a definite formation and arrangement. 

 Then there are difficulties in that most animals are bilaterally or 



