Chap. VIL] THEOEY OF NATUEAL SELECTION. 311 



over the whole suface of a marsupial sack secreted a 

 nutritious fluid ; and that these glands were improved 

 in function through natural selection, and concentrated 

 into a confined area, in which case they would have 

 formed a mamma. There is no more difficulty in under- 

 standing how the branched spines of some ancient Echi- 

 noderni, which served as a defence, became developed 

 through natural selection into tridactyle pedicellarise, 

 than in understanding the development of the pincers 

 of crustaceans, through slight, serviceable modifications 

 in the ultimate and penultimate segments of a limb, 

 which was at first used solely for locomotion. In the 

 avicularia and vibracula of the Polyzoa 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 origin- 

 ally served to tie together the pollen-grains, can be 

 traced cohering into caudicles ; and the steps can like- 

 wise 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 attached to the free ends of the caudicles ; — all 

 these gradations being 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 unreasonable to expect a 

 precise answer to such questions, considering our ignor- 

 ance of the past history of each species, and of the condi- 

 tions which at the present day determine its numbers and 

 15 



