1871.] 311 [Wilder. 



multiples are the typical numbers of acalephs, we find those which 

 have five or six spheromeres and other numerical combinations. We 

 need not therefore hesitate to compare an Aurelia with a quadripar- 

 tite and an Echinarachinus with a quinquepartite arrangement of 

 parts;" again, (200, 4, 379) "as soon as we can free ourselves from 

 the belief that histological complication and structural differentiation 

 are positive tests of homological relationships, and as soon as we al- 

 low full weight to embryological evidence, the close affinities of the 

 echinoderms and the other classes of radiates becomes self-evident." 



Spencer uses the following very suggestive language, which I ac- 

 cept as true, omitting his conclusion as to the cause of the superin- 

 duced segmentation, (299, 2, 110) ; "The parts composing the 

 supposed archetypal vertebras" (of Owen) "are constant neither in 

 their number nor in their relative" (natural) "position, nor in their 

 modes of ossification, nor in the separateness of their several individ- 

 ualities when present; .... everything goes to show that the 

 segmental composition which characterizes the apparatus of external 

 relation in most vertebrates is functionally determined or adaptive." 



Finally, Thomas Bell remarks, "the laws which regulate the nu- 

 merical variations in the different systems of organs in an animal, 

 are perhaps less defined or at least less understood than those which 

 relate to many other conditions of their existence. 1 ' 1 



Coming now to our special point, we may enumerate the morphical 

 relations of the digits as follows, taking the medius for an example, 

 since there has never been a doubt respecting its homology with ter- 

 tius, and both these are present in every known manus. 



1. Its special or plural homology with the medius of other Mam- 

 malia. (Fig. 2, A-B) 



2. Its single, serial and longal homology or mekesyntropy, with its 

 fellow-digits of the same manus. (C-D). 



3. Its single and vertical homology, hypsetropy with the medius of 

 an individual of the opposite sex. (E-F). 



4. Its single and lateral homology, platetropy with the medius of 

 the opposite side. (G-H). 



5. Its single and longal homology, meketropy, with the middle 

 dactyl, tertius. (G-I). 



Now although all these five relations are between a single digit 

 and another digit or dactyl, yet the relations of the several regions 



i Trans. Zool. Soc, vol. i, p. 133, 1833. 



