66 The Mycetozoa, and 



another, but often amongst several series, even of the 

 same family. It not unfrequently happens that a 

 member of one group presents a greater similarity to 

 its isomorph in another group with which it has no 

 relationship than it does to any other member of its 

 own group. Take a familiar illustration : suppose the 

 fingers of the two hands to represent the modifications 

 (species) of two such parallel types of Foraminifera : the 

 thumb of one hand resembles more closely the thumb of 

 the other hand than it does any other of the fingers of 

 its own." 



A comparison of the marsupial quadrupeds of Australia 

 and South America with the placental mammals of the rest 

 of the world presents another series of these isomorphs. 

 There are certain Marsupials which seem set over against 

 the Garni vora, others against the Eodents, and so forth. 

 Mr. Murray, in his "Geographical Distribution of Mam- 

 mals," has figured on the same page two animals, one a 

 small placental mouse, and the other a small marsupial 

 mouse, and their outward forms are almost indistinguishable ; 

 and yet the common parent of the two forms must be sought, 

 according to our present notions of phylogeny, before the 

 separation of the two great groups of Quadrupeds. 



Another instance of isomorphs occurs in the two parallel 

 groups of the Iridece and the Liliacece. Every one knows 

 how closely similar in outward appearance are the purple 

 crocus of the spring and the purple colchicum of the autumn ; 

 and yet the crocus is more nearly related to the yellow 

 iris than to the colchicum ; and the colchicum is more 



