THE FACTORS OP ORGANIC EVOLUTION. 7 



are homologous), this change could have arisen. In like 

 manner, the odd inflatable bag of the bladder-nosed seal, the 

 curious fishing-rod with its worm-like appendage carried on 

 the head of the lophius or angler, the spurs on the wings of 

 certain birds, the weapons- of the sword-fish and saw-fish, 

 the wattles of fowls, and numberless such peculiar struc 

 tures, though by no possibility explicable as due to effects 

 of use or disuse, are explicable as resulting from natural 

 ^ selection operating in one or other way. 



^Y i n the second place, while showing us how there have \ 

 arisen countless modifications in the forms, structures, ; 

 and colours of each part, Mr. Darwin has shown us how, 

 by the establishment of favourable variations, there may 

 arise new parts.^ Though the first step in the production i 

 of horns on the heads of various herbivorous animals, may j 

 have been the growth of callosities consequent on the-- 

 habit of butting such callosities thus functionally initiated 

 being afterwards developed in the most advantageous ways 

 by selection ; yet no explanation can be thus given of the 

 sudden appearance of a duplicate set of horns, as occasion 

 ally happens in sheep : an addition which, where it proved 

 beneficial, might readily be made a permanent trait by^ 

 natural selection. Again, the modifications which follow use 

 and disuse can by no possibility account for changes in the 

 numbers of vertebras ; but after, recognizing spontaneous, 

 or rather fortuitous, variation as a factor, we can see 

 that where an additional vertebra hence resulting (as 

 in some pigeons) proves beneficial, -survival of the fittest 

 may make it a constant character ; j and there may, by 

 further like additions, be produced extremely long strings 

 of vertebrae, such as snakes show us. Similarly with the 

 mammary glands. It is not an unreasonable supposition 

 that by the effects of greater or less function, inherited 

 through successive generations, these may be enlarged or 

 diminished in size ; but it is out of the question to allege 

 such a cause for changes in their numbers. There is no 



