INTRODUCTION 33 



homoplasy affects not only separate organs but entire types of animals, 

 groups of families and entire groups of orders, in a manner often extremely 

 confusing to the seeker of real ancestral relationships. Analogy, or like- 

 ness of function, through its power to transform unlike and unrelated 

 mammals or unlike and unrelated parts of mammals into likeness, has per- 

 formed such miracles that the inference of kinship or descent is often irre- 

 sistible; yet it is now well understood that a deeper ancestral resemblance 

 may closely relate animals which are externally dissimilar, while it may 

 just as widely separate animals which are externally similar. Similar desert 

 or steppe environments have fashioned the African jumping hare (Pedetes) 

 of the Cape, the true jerboas {Dipus, Alaciagn) of the steppes of Asia, and 

 the American jumping mice (Zapus) into similar saltatorial forms, yet 

 these are partly independent transformations. These jumping mice (Di- 

 podidffi) are paralleled by many forms: among other rodents by the 

 Heteromyidse (pocket mice), among the insectivores by the elephant 

 shrews (macroscelids), among marsupials by the rat-kangaroos (Bettongia) , 

 etc. 



Thus analogous adaptation is the counterforce to divergence, and 

 strongly tends to bring mammals together. Nevertheless two very im- 

 portant exceptions are to be noted. First, we rarely find exactly and 

 precisely the same means adopted in several groups of organs twice over; 

 and second, all the externally similar forms may be found on close exam- 

 ination to bear record of real internal and ancestral differences. Thus the 

 marsupial mole Notoryctes closely parallels the placental mole Talpa in 

 external appearance, but in its internal structure and dentition, in its mode 

 of reproduction, and in its skeleton it is fundamentally different. Thus 

 similarity of adaptation can never be mistaken by the close and logical 

 student of anatomy for similarity of descent or of ancestry. Of these 

 two kinds of adaptation and genetic resemblance, analogy is the woof, 

 composed of the horizontal strands which tie animals together by their 

 superficial resemblances, while homogeny (homology as applied to organs) 

 is the warp, composed of the vertical, hereditary strands which connect 

 animals with their ancestors and their successors. 



The grander applications of analogy to the groups of mammals were 

 first observed by Buffon in similar adaptations of animals evolving on 

 different continents. In earlier studies of the marsupials of Australia 

 Geoff roy St. Hilaire, De Blainville, and Richard Ow(^n observed the re- 

 markal)le analogies between the "families" into which these mammals 

 are divided and the "orders" of the northern continents. Cope ^ also ob- 

 served this grand mimicry of marsupial and placental orders. 



More recently it has been discovered that the collective mammals of 

 ancient South America, although of partly independent stock, in many 

 ways mimic the collective mammals of North America in Caenozoic times. 



* Cope, E. D., Origin of the Fittest. Essays on Evolution. 8vo, New York, 1887. 



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