OBJECTIONS TO THE THEORY OF NATURAL SELECTION 203 



defence, became developed through natural selection into tridactyle 

 pedicellariae, than in understanding the development of the pin- 

 cers of crustaceans through slight, serviceable modifications in the 

 ultimate and penultimate segments of a limb which was first used 

 solely for locomotion. In the avicularia and vibracula of the Poly- 

 zoa we have organs widely different in appearance developed from 

 the same source; and with the vibracula we can understand how 

 the successive gradations might have been of service. With the 

 pollinia of orchids, the threads which originally served to tie to- 

 gether the pollen grains can be traced cohering into caudicles; 

 and the steps can likewise be followed by which viscid matter, 

 such as that secreted by the stigmas of ordinary flowers, and still 

 subserving nearly but not quite the same purpose, became at- 

 tached to the free ends of the caudicles — all these gradations be- 

 ing of manifest benefit to the plants in question. With respect to 

 climbing plants, I need not repeat what has been so lately said. 



It has often been asked, if natural selection be so potent, why 

 has not this or that structure been gained by certain species, to 

 which it would apparently have been advantageous? But it is un- 

 reasonable to expect a precise answer to such questions, consid- 

 ering our ignorance of the past history of each species, and of the 

 conditions which at the present day determine its numbers and 

 range. In most cases only general reasons, but in some few cases 

 special reasons, can be assigned. Thus, to adapt a species to new 

 habits of life, many co-ordinated modifications are almost in- 

 dispensable, and it may often have happened that the requisite 

 parts did not vary in the right manner or to the right degree. Many 

 species must have been prevented from increasing in numbers 

 through destructive agencies, which stood in no relation to certain 

 structures, which we imagine would have been gained through 

 natural selection from appearing to us advantageous tG the species. 

 In this case, as the struggle for life did not depend on such struc- 

 tures, they could not have been acquired through natural selection. 

 In many cases complex and long-enduring conditions, often of a 

 peculiar nature, are necessary for the development of a structure; 

 and the requisite conditions may seldom have concurred. The be- 

 lief that any given structure, which we think, often erroneously, 

 would have been beneficial to a species, would have been gained 

 under all circumstances through natural selection, is opposed to 

 what we can understand of its manner of action. Mr. Mivart does 

 not deny that natural selection has effected something; but he 

 considers it as "demonstrably insufficient" to account for the phe- 

 nomena which I explain by its agency. His chief arguments have 



