360 STUDIES, SCIENTIFIC AND SOCIAL chap. 



of this kind can only have led to specific changes at 

 enormously long intervals, and that, as a general rule, they 

 can have had nothing whatever to do with the origin of an 

 overwhelming majority of living species. 



First, we have the four limbs of vertebrates, which 

 among all the marvellous variety of form and function, on 

 land, in the water, or in the air, is never exceeded, and 

 appears to have been fixed at a very early stage of the 

 development of the vertebrate type. Equally fixed, and 

 extending through a still vaster range of modifications of 

 specific forms, are the six legs and four wings of true 

 insects, which, as in vertebrates, may be reduced but never 

 increased in number. Still more extraordinary, because 

 less obviously connected with the main structure and 

 functions of the organism, is the limitation and per- 

 manence in the number of the subdivisions of limbs and 

 other appendages. There is no obvious reason why in 

 land -vertebrates the divisions of the hand and foot should 

 never exceed five, yet, not only is this number the maxi- 

 mum, but it may be considered the normal number of 

 which all others are reductions, since it still prevails 

 largely in the marsupials, rodents, carnivores, primates, 

 and lizards ; and the five-toed land-vertebrates (excluding 

 birds) are probably far more numerous than those with a 

 lesser number. 



In birds there are only four toes as a maximum, and 

 comparatively few have a smaller number. But we have 

 here a peculiarity in the numbers of the toe- joints which 

 does not occur in any other vertebrates. These form a 

 series in arithmetical progression, the hind toe having two, 

 and the others three, four, and five joints in regular order ; 

 and this rule is very nearly universal, the only exceptions 

 being in some of the swifts and goatsuckers, whose habits 

 render the feet of comparatively little importance, while 

 their general organisation is of a somewhat low type. 



Coming to insects, we again find the legs consisting of a 

 limited number of parts, and, strangely enough, this 

 number is again five — the coxa, trochanter, femur, tibia, 

 and tarsus. The tarsus, however, is subdivided into small 

 movable joints, and these, too, are five as a maximum, but 



