356 THE VARIATION OF ANIMALS IN NATURE 



and, in fact, it can only lumber along in an ungainly fashion, 

 trailing its prehensile tail behind it. The faculty of tree- 

 climbing is still retained and employed if the opossum is 

 brought up to a tree. Yet the animal ranges with apparent 

 success over enormous treeless areas in the Argentine. 

 Evidently lack of specialisation in some respects has been 

 able to atone for it in others. 



In insects we may cite the familiar case of the Chermesidae, 

 which under normal conditions have a very complicated life- 

 cycle spent on two species of coniferous host plants. All would 

 agree that the cycle, including numerous different types of 

 individuals, with migration from one species of tree to another, 

 was in a broad way highly adaptive. Yet where one host is 

 absent, as in the case of some English Chermesidae, the life- 

 cycle is passed on one tree only (so-called anholocyclic life- 

 history), and certain types of individuals, including as a rule 

 the sexual forms, are no longer produced. It is difficult to 

 reconcile this very elastic power of response with the idea of 

 any detailed adaptation in the original state. 



These examples are really complementary to the fact that 

 the same modifications may be found in animals leading 

 quite different lives. This point has been well illustrated 

 in a number of groups of vertebrates by Guyenot (1930, 

 pp. 265-79). One of the most extraordinary instances is the 

 parallelism in a number of characters between the Cetacea 

 and the Edentata. Certainly not all of these characters are 

 very obviously adaptive, but some of them have been claimed 

 to be so in one group or another. The following peculiarities 

 are known to occur in one or more genera of both groups : 

 ' retia mirabilia ' in the tail and legs ; presence of two venae 

 cavae and absence of the azygos ; pterygoids forming fused 

 palatines meeting in the median line and extending posteriorly 

 to the opening of the fauces ; feeble mandible without a 

 coronoid process ; double articulation of the ribs with 

 the sternum ; ribs unusually broad ; absence of the bile 

 reservoir. 



We can see an analogous phenomenon in the wide range 

 of country inhabited by many species. We are all familiar 

 with species which range over areas in Europe including 

 climatic and edaphic conditions of very varied types. Even 

 within a small area there may be a wide range of conditions, 



