THE METHOD OP ORGANIC EVOLUTION. 423 



types of life at all events, the most stable of all characters, and the 

 most permanent during long periods of evolution, and througliout 

 changes which have led to the production of a marvelous variety and 

 abundance of specific forms, are these very characters of the number 

 and relative positions of serial organs; whence it follows that varia- 

 tions 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 tlie four limbs of vertebrates, which, among all the 

 marvelous 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 extraordi- 

 nary, because less obviously connected with the main structure and 

 functions of the organism, is the limitation and permanence in the num- 

 ber 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 tlie nornml 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 num- 

 bers 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 compara- 

 tively little importance, while their general organization is of a some- 

 what low type. 



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

 ber 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 in 

 certain groups are reduced to four, three, or two. The five-jointed tarsus 

 is, however, the most prevalent, and in the enormous order of Coleoptera 

 or beetles, comprising at least one hundred thousand described species, 

 fully half belong to families which have the tarsi five-jointed. Even the 

 antennae, although they vary greatly in the number of joints, yet in 

 numerous large groups comprising many thousands of species, they 

 have the number of joints constant. Another indication of the tendency 

 of serial parts to become fixed in number, is the typical limitation of the 



