CONVERGENT EVOLUTION 443 



by certain " annectent " forms known as " Carrion-hawks " ; and 

 these, by a parity of reasoning, join together also the Cariama. 



The Carrion-hawks, which number several species, are 

 natives of South America. They are really close allies of the 

 Falcons, but, as might be supposed from the connection here, 

 they are long-legged birds which run much upon the ground, 

 in which particulars they differ conspicuously from true Falcons. 



So close is the resemblance between these forms— Cariamas, 

 Secretary-birds and Carrion-hawks — that Ornithologists so dis- 

 tinguished as Dr. Sharpe have regarded them as close allies, 

 uniting them to form a single sub-family ! 



The dissecting knife has, however, severed the union, and at 

 the same time made it necessary to find an explanation of the 

 strange likenesses. This now appears to be found in "Con- 

 vergence ". These several types may be called " homoplasts " ; 

 they are the outcome of the working of " homoplasy ". 



None obtain their food after the fashion of the true Hawks 

 — by seizing swiftly moving prey by the feet while on the 

 wing; but all hunt comparatively slowly moving animals — 

 snakes, mice, frogs, large insects, etc. ; and many subsist partly 

 on Carrion. This accounts for the long legs and relatively 

 short toes — the latter indeed in the Secretary-bird are very 

 short. Living on terrestrial prey easily overtaken by running, 

 the legs have in consequence gradually lengthened, and the 

 necks have had to keep pace therewith. 



Hardly less striking is the resemblance between the Crane 

 and Stork tribe. So real is this that only those with some 

 knowledge of Ornithology can distinguish the one from the 

 other. Yet, here again, is an instance of homoplasy, and not 

 of parallelism. The two groups have independent origins, and 

 have acquired this resemblance to one another by the action of 

 " similar forces or environment " on parts which are exactly or 

 nearly alike. 



The familiar White Stork, or still better, the Common Heron, 

 may serve as a type of the Stork tribe — birds with long, pointed 

 beaks, and long neck and legs. In the same terms one may 

 describe the Cranes, though a few species have short beaks. 



Though superficially so much alike, and though both affect 

 the same haunts — swamps and fens — the food they seek there 

 is different. The Stork appears to require an animal diet 



