CONVERGENCE IN EVOLUTION 261 



It is limited in its acquisitions by what has been 

 already established, just as an architect in making 

 an addition to a house is limited by the existing 

 style. Flying birds differ greatly in detail, but 

 within a relatively narrow range. No mutation 

 can be considered, so to speak, which risks a loss 

 of the secret of flight. Along this line of thought 

 we may get another gleam of light on convergence. 

 Other solutions than that adopted are possible to a 

 theoretical organism, but not to this or that par- 

 ticular organism which has to run in the trammels 

 of a hereditary organization. Why should all the 

 ordinary mammals of our acquaintance be restricted 

 to four legs? The dachshund often looks as if it 

 could do with another pair about half-way along, 

 and so does a sow also a somewhat artificial 

 creature. The proposition of an extra pair cannot 

 be considered, however, for higher vertebrates are 

 hereditarily tied down to a maximum of two pairs of 

 limb-buds, just as they probably are to a maximum 

 of twelve cranial nerves, where an increase would be 

 more readily effected. It is interesting, however, 

 to force the question back and ask why some of the 

 lower vertebrates, notably fishes, might not have 

 tried the experiment of an additional pair of limbs. 

 In all probability the answer is simple : that paired 

 limbs were established at the level of fishes, and 

 that their primary function was in connection with 

 balancing, not with locomotion, the locomotor 

 organ in almost all fishes being the so-called tail. 

 Now, for balancing purposes the choice is between 



