THK MATUACHIA OF NOKTH AMERICA. 



249 



The fjunilies of Arcifera with opi.sthoc^i'louK vt'itebitB are omitted 

 from tiie table as having no eounterpart amonj; tlie Firmisternin. 



Tiiese two series are what I liave (^iiMed " homoloj^ous };'i'onp8," and 

 the eoirespondiiiy genera " heteroh){j;ons terms. 1 have also supposed 

 that one sucii series may have been derived from the other, in evohi- 

 tion, by a ehange in the one eharaeter which <listinguish<'s the two 

 series. In the ease <tf some homoh)gous series it is not iinliJiely tliat 

 this may have taken phuie, but it is neeessary, in order to be snre tliat 

 sueh has been tlie i)rocess of evohition, to distinguish between two dif- 

 ferent kinds of homohigoiis gronjjs. In one kind the parallel eharac- 

 teraof theone group may have been derived from those of the other by 

 descent, according to the priiH'iple called by Laidcester " homogeny." 

 In the other case, like modifications of structure have arisen in differ- 

 ent series of animals as a result of the operation of similar energies, as 

 that <>f the animal an«l itsenvironnaMit. This is the principle of "homo- 

 l)lassy." To the latter kind belong the imitations fonn«l to exist be- 

 twtuMi tlu^ placental and didelphian mammalia. The relation between 

 the aiciferous and lirmisternial Amira may be one of homogeny. We 

 may then parallelize tht^ families which may exhibit true cases of de- 

 scent as follows : 



FlIiMISTKKNIA. 



I'liryiiiscid;!'. 



])('iHli'(>li:itiilii\ 



])y)-<-<>|i)ii<l;r. 



RaiiiiiM'. 



CiT.idiliatracliidio. 



Ahcikkra. 



Hiif(ini<l:i>. 



DiMnlropIiiyiiisiidif. 



lVl<nl,vti(lii'. 



(\VStifr|lUtllill!l'. 



]|<'ini|ilir:i(;ti<l;c. 



It is, however, |)robal)le that the I'elodytida' is the generalized form 

 from wlii<'h most of the arciferons families have been deiive<l ; and it was 

 itself probably a descendant of the families with opisthoco'lous verte- 

 bra', as already indi<'ated. 



The Kaiiida' embracu's many genera which imitate in details many 

 genera of Arcifera. The metropolis of the former, as of the LacvrtUia 

 acrodontHy is the rifi'n* VnUcotropiva, while tin' latter liav*' but tew 

 representatives out of tin' A*. /»'. Svolrophut and Aiistralis, where but one 

 or two spi'cies of the foi'mer o(;enr. In both \\v can trace a series in 

 which the outer metatarsal is gradually liberated from thi^ penultimate, 

 to afl'onl greater extension for the web in the most a<piatic tyjx's, and 

 among those where these boiu's are bound, from wi'bless to webbed 

 types. In both we have burrowing and arboreal genera. 



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