THE CLASSIFICATION OF BIRDS 21 



their diagnosis. No one of these three subclasses appears to 

 possess as many diagnostic characters or combinations of characters 

 as the two other subclasses ; but perhaps it ought to be a matter 

 of congratulation that they are capable of being diagnosed at all. 

 Though the characters are only two in number, they appear to be 

 extremely difficult of independent acquirement. They are the 

 loss of the fifth secondary and the abnormal arrangement of the 

 deep plantar tendons. 



In regarding the arrangement of the deep plantar tendons 

 from a taxonomic point of view, the flexor longus hallucis may be 

 passed over as of little importance. The hallux is so often absent 

 or has ceased to be functional, that its muscles may vary in closely 

 allied genera, whilst the flexor perforans diyilorum is constant in 

 every family which has been examined.* 



What possible advantage could be gained by disturbing the 

 normal arrangement of the plantar?, or by what possible method- 

 the change from the normal condition was arranged, are problems 

 so utterly insoluble that it requires an abnormal amount of faith 

 in the theory of Natural Selection to suppose that no other cause 

 for the evolution was operating. 



The loss of the fifth secondary is a character which appears to 

 be of greater taxonomic importance than an abnormal arrange- 

 ment of the deep plantar tendons. No suggestion of any 



* The flexor perforans diyitorum is modified in four different ways, which may 

 be remembered most easily by numbering them according to the number of 

 d'gits to which the muscle leads. 



Variation No. 1 : the Picine modification is the most simple arrangement, as 

 the front plantar leads to one digit only (the third). It is peculiar to the Scan- 

 sores, a small group of birds composed of the families Picidce, Capitonidte, Gal- 

 lu'ida;, and Rhampkastidce. 



Variation No. 2 : the Trogonine arrangement characterises the few birds in 

 which the front plantar leads to two digits only (third and fourth), and is 

 believed to be peculiar to the Troyonidce. 



Variation No. 3 : the Passerine arrangement is the normal one. The front 

 plantar leads to three digits (the second, third, and fourth), except of course in 

 the very rare instances where one of the front digits is absent. 



Variation No. 4 : the Picarian modification is perhaps the most abnormal, 

 the front plantar leading to all four digits, and the hallux, which is always 

 present, being unconnected with the flexor lonyus hallucis. This arrangement is 

 found only in the Coliidce, Alcedinida>, Momotidcc, Todidce,'Cyfsclidce, Caprimul- 

 gid(f, Podargidcc, Bucerotidce, Steatornithidce, Coraciidce, and Meropidcc. 



Variation No. 5 : the Pseudo-Gryphine modification is unique. The front 

 plantar leads to three digits (the first, second, and third), whilst the hind 

 plantar leads sometimes to three digits (the second, third, and fourth), and some- 

 times to only two (third and fourth). So far as I know, this arrargement is 

 peculiar to the Cath'.irtidcc. 



