NATURAL SCIENCES OF PHILADELPHIA. 281 



at. Of homologous groups. 



Naturally followiag the admissioa of a developmental succession of organic 

 beings, is the question of its relation to the different surfaces of land and 

 water on the earth. The following considerations bear on this subject. 



Among the higher groups of animals can be detected series " homologous " 

 on the same principle as the alcohols (? compound radicals) and their deriva- 

 tives ; and the component types of each can be, and have been in many in- 

 stances, shown to be "heterologous," as are the ethers, mercaptans, aldehydes, 

 acids, etc. Among Mammalia two partly homologous series have been pointed 

 out, Implacentialia and Placentialia ;* possibly such are the types Altrices and 

 Pracaeoces among Aves ; of a lesser grade in this class are the parallel series 

 of Pullastra; and Galliuaj; of Clamatores and Osciues. Among Tortoises I 

 have alluded to the Pleurodira as compared with the remainder of the order, 

 already parallelized by Wagler ; and of lesser grades, the series among Lacer- 

 tilia of Acrodouta and Iguania, parallelized by Dumeril and Bibron, and of 

 Teid;« and Lacertida3, compared by Wiegmann. I have discovered a full paral- 

 lelism between the Raniform and Arciferous Anura. It is carried out between 

 the Characiuiand a group of remaining Physostomous Fishes, perhaps not yet 

 well delined ; it is exhibited between the orders Diptera and Hymenoptera 

 among insects. None of these comparisons can be allowed, of course, without 

 the most searching anatomical and embryological analysis. 



This heterology is what Swaiuson and others called " analogy " as dis- 

 tinguished from affinity. It generally relates genera of different zoological 

 regions. Mimetic analogy, on the contrary, relates genera of the same region ; 

 it is a superficial imitation which has occurred to critical biologists, and is of 

 much interest, though as yet but little investigated. It has as yet been ob- 

 served in external characters only, but occurs in internal also ; it has been 

 accounted for in the first case by the supposed immunity from enemies arising 

 from resemblance to well defended types. No such explanation will, however, 

 answer in the latter case. I believe such coincidences express merely the de- 

 velopmental type common to many heterologous series of a given Zoological 

 " Region ;" this will be alluded to a few pages later. 



We naturally inquire, is there anything in the food, the vegetation, or the 

 temperature to account for this apparent diversity in the different regions ? 

 Are there not carnivora, herbivora, seed-eaters, insectivores, and tree climbers, 

 where game and grass, seeds and insects and forests grow the world over? 

 We answer undoubtedly there are, and these adaptations to food and climate 

 are indeed as nothing in the general plan of creation, for every type of every 

 age has performed these functions successively. 



(i. Of Heterology. 



This relation will be exhibited by a few examples from groups knowu to 

 the writer, commencing with the Batrachia Anura. 



Raniformes. Arciferi. 



External metatarsal free. 



Aquatic. Rana. Pseudis. 



Metatars. shovel. Hoplobatrachus. Mixophyes. 



External metatarsal attached. • r 



Feet webbed. 



Metatars. shovel. Pyxicephalus. Tomopterna. 



Arboreal ; vom. teeth. Leptopelis. Hyla. 



" no " " Hyperolius. Hyleila. 



Subarboreal. Hylambates. Nototvema. 



Feet not webbed. 



Terrestrial. Cassina. Cystignathua. 



" spurred, Hemimantis. Gomphobates. 



1868.] 19 



